The Progressive Corporation (PGR) 10-Q 3 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	3 Months Ended
	Mar. 31, 2017shares
Document Document And Entity Information [Abstract]
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	80,661
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Amendment Flag	false
Trading Symbol	PGR
Entity Common Stock, Shares Outstanding	580,902,022

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Revenues
Net premiums earned		$ 6,026.7	$ 5,317.4
Period over prior year period percent change in net premiums earned		13.00%
Investment income		$ 129.2	118.8
Period over prior year period percent change in investment income		9.00%
Net realized gains (losses) on securities:
Net impairment losses recognized in earnings		$ (1)	0
Net realized gains (losses) on securities		$ 52.9	17.4
Period over prior year period percent change in net realized gains (losses) on securities		204.00%
Total net realized gains (losses) on securities		$ 51.9	17.4
Period over prior year period percent change in total net realized gains (losses) on securities		198.00%
Fees and other revenues		$ 85.2	78.9
Period over prior year period percent change in fees and other revenues		8.00%
Service revenues		$ 28.5	25
Period over prior year period percent change in service revenues		14.00%
Gains on extinguishment of debt		$ 0.2	0
Total revenues		$ 6,321.7	5,557.5
Period over prior year period percent change in total revenues		14.00%
Expenses
Losses and loss adjustment expenses		$ 4,263.4	3,913.4
Period over prior year period percent change in losses and loss adjustment expenses		9.00%
Policy acquisition costs		$ 502.9	440.3
Period over prior year period percent change in policy acquisition costs		14.00%
Other underwriting expenses		$ 845.6	755.8
Period over prior year period percent change in other underwriting expenses		12.00%
Investment expenses		$ 5.6	4.8
Period over prior year period percent change in investment expenses		17.00%
Service expenses		$ 25.9	21.6
Period over prior year period percent change in service expenses		20.00%
Interest expense		$ 36.8	34.2
Period over prior year period percent change in interest expense		8.00%
Total expenses		$ 5,680.2	5,170.1
Period over prior year period percent change in total expenses		10.00%
Net Income
Income before income taxes		$ 641.5	387.4
Period over prior year period percent change in income before income taxes		66.00%
Provision for income taxes		$ 211.2	128.7
Period over prior year period percent change in provision for income taxes		64.00%
Net income		$ 430.3	258.7
Period over prior year period percent change in net income		66.00%
Net (income) loss attributable to noncontrolling interest (NCI)		$ (6)	(0.5)
Net income attributable to Progressive		$ 424.3	258.2
Period over prior year period percent change in net income attributable to Progressive		64.00%
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on securities		$ 132.4	69.5
Period over prior year period percent change in total net unrealized gains (losses) on securities		91.00%
Net unrealized losses on forecasted transactions		$ 2.3	(0.3)
Foreign currency translation adjustment		$ 0.3	0.5
Period over prior year period percent change in foreign currency translation adjustment		(40.00%)
Other comprehensive income		$ 135	69.7
Period over prior year period percent change in other comprehensive income		94.00%
Other comprehensive (income) loss attributable to NCI		$ (0.9)	(2.1)
Period over prior year period changein Comprehensive (income) loss attributable to NCI, net of tax		(57.00%)
Comprehensive income attributable to Progressive		$ 558.4	$ 325.8
Period over prior year period percent change in comprehensive income		71.00%
Computation of Per Share Earnings Attributable to Progressive
Average shares outstanding - Basic		580.3	583.2
Period over prior year period percent change in average shares outstanding - basic		0.00%
Net effect of dilutive stock-based compensation		3	2.3
Period over prior year period percent change in net effect of dilutive stock-based compensation		30.00%
Total average equivalent shares - Diluted		583.3	585.5
Period over prior year period percent change in total equivalent shares - diluted		0.00%
Basic: Earnings per share		$ 0.73	$ 0.44
Period over prior year period percent change in basic: net income per share		65.00%
Diluted: Earnings per share		$ 0.73	0.44
Period over prior year period percent change in diluted: net income per share		65.00%
Dividends declared per share	[1]	$ 0	$ 0
[1]	Progressive maintains an annual dividend program. See Note 9 – Dividends for further discussion.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Investments - Available-for-sale, at fair value:
Fixed maturities (amortized cost: $17,444.8, $13,892.3, and $16,287.1)		$ 17,474.3	$ 16,243.8	$ 13,966.4
Equity securities:
Nonredeemable preferred stocks (cost: $710.3, $731.9, and $734.2)		809.8	853.5	832.2
Common equities (cost: $1,451.5, $1,524.0, and $1,437.5)		2,978.1	2,812.4	2,705
Short-term investments (amortized cost: $3,040.8, $3,826.9, and $3,572.9)		3,040.8	3,572.9	3,826.9
Total investments		24,303	23,482.6	21,330.5
Cash		214.5	211.5	203.9
Restricted cash1	[1]	0.5	14.9	6
Accrued investment income		110.1	103.9	93.6
Premiums receivable, net of allowance for doubtful accounts of $174.5, $157.6, and $186.8		4,850.7	4,509.2	4,378.9
Reinsurance recoverables, including $88.6, $59.5, and $83.8 on paid losses and loss adjustment expenses		1,939	1,884.8	1,568.2
Prepaid reinsurance premiums		182.7	170.5	216.7
Deferred acquisition costs		679.5	651.2	600
Property and equipment, net of accumulated depreciation of $881.7, $799.6, and $845.8		1,166	1,177.1	1,061.9
Goodwill		449.4	449.4	447.6
Intangible assets, net of accumulated amortization of $125.0, $62.9, and $109.5		417.3	432.8	479.4
Other assets		497.8	339.6	334.9
Total assets		34,810.5	33,427.5	30,721.6
Liabilities
Unearned premiums		7,945	7,468.3	7,140.7
Loss and loss adjustment expense reserves		11,628.9	11,368	10,286.7
Net deferred income taxes		168.4	111.3	113.9
Dividends payable		0	395.4	0
Accounts payable, accrued expenses, and other liabilities		2,939.4	2,495.5	2,451.4
Debt	[2]	3,111.7	3,148.2	2,701.6
Total liabilities		25,793.4	24,986.7	22,694.3
Redeemable noncontrolling interest (NCI)3	[3]	494.2	483.7	467.4
Common shares, $1.00 par value (authorized 900.0; issued 797.5, 797.6, and 797.5 including treasury shares of 216.6, 214.6, and 217.6)		580.9	579.9	583
Paid-in capital		1,331.1	1,303.4	1,231.3
Retained earnings		5,543.4	5,140.4	4,877.6
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities		1,072	939.6	878.5
Net unrealized losses on forecasted transactions		(7.1)	(9.4)	(8.5)
Foreign currency translation adjustment		(0.8)	(1.1)	(1)
Accumulated other comprehensive (income) loss attributable to NCI		3.4	4.3	(1)
Total accumulated other comprehensive income attributable to Progressive		1,067.5	933.4	868
Total shareholders’ equity		8,522.9	7,957.1	7,559.9
Total liabilities, redeemable NCI, and shareholders’ equity		$ 34,810.5	$ 33,427.5	$ 30,721.6
[1]	See Note 7 – Supplemental Cash Flow Information for further discussion.
[2]	Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.
[3]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 17,444.8	$ 16,287.1	$ 13,892.3
Nonredeemable preferred stocks, cost	710.3	734.2	731.9
Common equities, cost	1,451.5	1,437.5	1,524
Short-term investments, amortized cost	3,040.8	3,572.9	3,826.9
Premiums receivable, allowance for doubtful accounts	174.5	186.8	157.6
Reinsurance recoverables, paid losses and loss adjustment expenses	88.6	83.8	59.5
Property and equipment, accumulated depreciation	881.7	845.8	799.6
Finite-Lived Intangible Assets, Accumulated Amortization	$ 125	$ 109.5	$ 62.9
Common Shares, par value (USD per share)	$ 1	$ 1	$ 1
Common Shares, authorized (shares)	900,000,000	900,000,000	900,000,000
Common Shares, issued (shares)	797,500,000	797,500,000	797,600,000
Common Shares, treasury shares (shares)	216,600,000	217,600,000	214,600,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions	Total	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	AOCI Attributable to Parent [Member]	Serial Preferred Shares [Member]	Voting Preference Shares [Member]
Balance, Beginning of period at Dec. 31, 2015		$ 583.6	$ 1,218.8	$ 4,686.6	$ 800.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive	$ 258.2			258.2
Tax benefit from vesting of equity-based compensation			6.6
Treasury shares purchased		(2.3)	(4.8)	(63.2)
Cash dividends declared on common shares				0.2
Net restricted equity awards (issued)/(vested)/forfeited		1.7	(1.7)
Amortization of equity-based compensation			12.8
Reinvested dividends on restricted stock units			(0.3)	0.3
Adjustment to carrying amount of redeemable noncontrolling interest	(0.1)		(0.1)
Attributable to noncontrolling interest	(2.1)				(2.1)
Other comprehensive income	67.6				69.7
Other, net				(4.5)
Balance, End of period at Mar. 31, 2016	7,559.9	583	1,231.3	4,877.6	868
Balance, Beginning of period at Dec. 31, 2015		583.6	1,218.8	4,686.6	800.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to carrying amount of redeemable noncontrolling interest	(4.2)
Attributable to noncontrolling interest	3.2
Balance, End of period at Dec. 31, 2016	$ 7,957.1	579.9	1,303.4	5,140.4	933.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares authorized (in shares)						20,000,000	5,000,000
Preferred stock, shares issued (in shares)						0	0
Preferred stock, shares outstanding (in shares)						0
Common Shares, par value (USD per share)	$ 1
Common Shares, par value (USD per share)	$ 1
Net income attributable to Progressive	$ 424.3			424.3
Tax benefit from vesting of equity-based compensation			0
Treasury shares purchased		(0.6)	(1.3)	(19.5)
Cash dividends declared on common shares				0
Net restricted equity awards (issued)/(vested)/forfeited		1.6	(1.6)
Amortization of equity-based compensation			30.5
Reinvested dividends on restricted stock units			0.3	(0.3)
Adjustment to carrying amount of redeemable noncontrolling interest	0.2		(0.2)
Attributable to noncontrolling interest	(0.9)				(0.9)
Other comprehensive income	134.1				135
Other, net				(1.5)
Balance, End of period at Mar. 31, 2017	$ 8,522.9	$ 580.9	$ 1,331.1	$ 5,543.4	$ 1,067.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares authorized (in shares)						20,000,000	5,000,000
Preferred stock, shares issued (in shares)						0	0
Preferred stock, shares outstanding (in shares)						0
Common Shares, par value (USD per share)	$ 1

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Cash Flows [Abstract]
Net income	$ 430.3	$ 258.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	41	27.5
Amortization of intangible assets	15.5	15.5
Net Amortization of fixed-income securities	22.1	20.5
Amortization of equity-based compensation	30.9	12.8
Net realized (gains) losses on securities	(51.9)	(17.4)
Net (gains) losses on disposition of property and equipment	1.3	0.1
(Gains) losses on extinguishment of debt	(0.2)	0
Changes in:
Premiums receivable	(341.4)	(391.1)
Reinsurance recoverables	(54.2)	(79.4)
Prepaid reinsurance premiums	(12.2)	(17.4)
Deferred acquisition costs	(28.3)	(35.9)
Income taxes	212.7	91.8
Unearned premiums	476.3	518.6
Loss and loss adjustment expense reserves	260.8	247.6
Accounts payable, accrued expenses, and other liabilities	137.4	200.1
Restricted cash	14.4	(5.7)
Other, net	(35.7)	(27.5)
Net cash provided by operating activities	1,118.8	818.8
Purchases:
Fixed maturities	(3,203.9)	(2,468.2)
Equity securities	(52.5)	(180.1)
Sales:
Fixed maturities	1,014.1	2,105.9
Equity securities	86.5	97.9
Maturities, paydowns, calls, and other:
Fixed maturities	1,006.5	1,829.8
Equity securities	26.6	0
Net sales (purchases) of short-term investments	536.8	(1,653)
Net unsettled security transactions	83.2	70.4
Collateral on derivative instruments	(136.1)	0
Purchases of property and equipment	(35)	(54.3)
Sales of property and equipment	11.2	2
Net cash used in investing activities	(662.6)	(249.6)
Cash Flows From Financing Activities
Proceeds from exercise of equity options	0.5	0
Tax benefit from vesting of equity-based compensation	0	6.6
Payments of debt	(6.2)	(6.8)
Reacquisition of debt	(30.7)	0
Dividends paid to shareholders	(395.4)	(519)
Acquisition of treasury shares for restricted stock tax liabilities	(21.4)	(18.9)
Acquisition of treasury shares acquired in open market	0	(51.4)
Net cash used in financing activities	(453.2)	(589.5)
Effect of exchange rate changes on cash	0	0.1
Increase (decrease) in cash	3	(20.2)
Cash, January 1	211.5	224.1
Cash, March 31	$ 214.5	$ 203.9

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation — The accompanying consolidated financial statements include the accounts of The Progressive Corporation and ARX Holding Corp. (ARX), and their respective wholly owned insurance and non-insurance subsidiaries and affiliates, in which Progressive or ARX has a controlling financial interest. The Progressive Corporation owned 69.0% of the outstanding capital stock of ARX at March 31, 2017 and 69.2% at March 31, 2016 and December 31, 2016 . The decrease reflects ARX employee stock options that were exercised during the first quarter 2017. All intercompany accounts and transactions are eliminated in consolidation. The consolidated financial statements reflect all normal recurring adjustments that, in the opinion of management, were necessary for a fair statement of the results for the interim periods presented. The results of operations for the period ended March 31, 2017 , are not necessarily indicative of the results expected for the full year. These consolidated financial statements and the notes thereto should be read in conjunction with Progressive’s audited financial statements and accompanying notes included in Exhibit 13 to our Annual Report on Form 10-K for the year ended December 31, 2016 (" 2016 Annual Report to Shareholders"). Other assets on the consolidated balance sheets include properties that are considered "held for sale," if any. At March 31, 2017, no properties were held for sale. At March 31, 2016 and December 31, 2016, the fair value of these properties, less the estimated cost to sell them, were $8.7 million .

Investments

Investments	3 Months Ended
	Mar. 31, 2017
Investments Schedule [Abstract]
Investments	Investments — Our securities are reported at fair value, with the changes in fair value of these securities (other than hybrid securities and derivative instruments) reported as a component of accumulated other comprehensive income, net of deferred income taxes. The changes in fair value of the hybrid securities and derivative instruments are recorded as a component of net realized gains (losses) on securities. The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. The net holding period gains (losses) represent the amounts realized on our hybrid securities only. ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2017 Fixed maturities: U.S. government obligations $ 3,345.5 $ 0.9 $ (20.6 ) $ 0 $ 3,325.8 13.7 % State and local government obligations 2,475.8 23.3 (10.7 ) 0 2,488.4 10.2 Foreign government obligations 22.9 0 0 0 22.9 0.1 Corporate debt securities 5,199.6 23.1 (15.0 ) 0.3 5,208.0 21.4 Residential mortgage-backed securities 1,393.9 24.6 (11.0 ) 2.1 1,409.6 5.8 Agency residential pass-through obligations 39.1 0.1 (0.7 ) 0 38.5 0.2 Commercial mortgage-backed securities 2,216.1 13.7 (16.3 ) 0 2,213.5 9.1 Other asset-backed securities 2,562.4 5.4 (3.1 ) 0.2 2,564.9 10.6 Redeemable preferred stocks 189.5 14.3 (1.1 ) 0 202.7 0.8 Total fixed maturities 17,444.8 105.4 (78.5 ) 2.6 17,474.3 71.9 Equity securities: Nonredeemable preferred stocks 710.3 111.3 (11.8 ) 0 809.8 3.3 Common equities 1,451.5 1,531.4 (4.8 ) 0 2,978.1 12.3 Short-term investments 3,040.8 0 0 0 3,040.8 12.5 Total portfolio 1,2 $ 22,647.4 $ 1,748.1 $ (95.1 ) $ 2.6 $ 24,303.0 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2016 Fixed maturities: U.S. government obligations $ 1,375.8 $ 12.2 $ 0 $ 0 $ 1,388.0 6.5 % State and local government obligations 2,452.0 57.4 (1.2 ) 0 2,508.2 11.8 Foreign government obligations 21.9 0 0 0 21.9 0.1 Corporate debt securities 3,795.2 50.6 (6.2 ) 0.4 3,840.0 18.0 Residential mortgage-backed securities 1,810.9 19.5 (28.5 ) (0.4 ) 1,801.5 8.4 Agency residential pass-through obligations 46.5 0.2 (0.1 ) 0 46.6 0.2 Commercial mortgage-backed securities 2,418.7 26.4 (18.9 ) 0 2,426.2 11.4 Other asset-backed securities 1,706.3 1.3 (4.2 ) 0.3 1,703.7 8.0 Redeemable preferred stocks 265.0 14.2 (48.9 ) 0 230.3 1.1 Total fixed maturities 13,892.3 181.8 (108.0 ) 0.3 13,966.4 65.5 Equity securities: Nonredeemable preferred stocks 731.9 124.7 (24.4 ) 0 832.2 3.9 Common equities 1,524.0 1,186.7 (5.7 ) 0 2,705.0 12.7 Short-term investments 3,826.9 0 0 0 3,826.9 17.9 Total portfolio 1,2 $ 19,975.1 $ 1,493.2 $ (138.1 ) $ 0.3 $ 21,330.5 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value December 31, 2016 Fixed maturities: U.S. government obligations $ 2,899.2 $ 0 $ (29.1 ) $ 0 $ 2,870.1 12.2 % State and local government obligations 2,509.5 13.8 (20.7 ) 0 2,502.6 10.7 Foreign government obligations 24.5 0 0 0 24.5 0.1 Corporate debt securities 4,557.8 17.3 (24.3 ) 0.1 4,550.9 19.4 Residential mortgage-backed securities 1,448.5 23.7 (15.0 ) 1.5 1,458.7 6.2 Agency residential pass-through obligations 41.2 0 (0.6 ) 0 40.6 0.2 Commercial mortgage-backed securities 2,266.9 12.0 (25.5 ) 0 2,253.4 9.6 Other asset-backed securities 2,350.7 4.6 (4.4 ) 0.2 2,351.1 10.0 Redeemable preferred stocks 188.8 5.1 (2.0 ) 0 191.9 0.8 Total fixed maturities 16,287.1 76.5 (121.6 ) 1.8 16,243.8 69.2 Equity securities: Nonredeemable preferred stocks 734.2 135.4 (16.1 ) 0 853.5 3.6 Common equities 1,437.5 1,377.0 (2.1 ) 0 2,812.4 12.0 Short-term investments 3,572.9 0 0 0 3,572.9 15.2 Total portfolio 1,2 $ 22,031.7 $ 1,588.9 $ (139.8 ) $ 1.8 $ 23,482.6 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions and collateral on open derivative positions; at March 31, 2017 and 2016 , and December 31, 2016 , we had $111.0 million , $47.3 million , and $27.8 million , respectively, included in "other liabilities." In addition, at March 31, 2017, we had $136.1 million of margin collateral on an open derivative position, which was included in "other assets." 2 The total fair value of the portfolio at March 31, 2017 and 2016 , and December 31, 2016 included $0.7 billion , $0.7 billion , and $1.3 billion , respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions. Short-Term Investments Our short-term investments may include commercial paper and other investments that are expected to mature within one year. We did not enter into any repurchase commitment transactions during the first three months of 2017 or 2016 , and we had no open repurchase commitments at March 31, 2017 and 2016, or December 31, 2016 . Also included in short-term investments are reverse repurchase commitment transactions, where we loan cash to approved counterparties and receive U.S. Treasury Notes pledged as collateral against the cash borrowed. Our exposure to credit risk is limited due to the nature of the collateral (i.e., U.S. Treasury Notes) received. We have counterparty exposure on these trades in the event of a counterparty default to the extent the collateral security's value is below the amount of cash we delivered to acquire the collateral. The short-term duration of the transactions (primarily overnight) reduces that exposure. We had no open reverse repurchase commitments at March 31, 2017 and 2016, or December 31, 2016 . We did not enter into any reverse repurchase commitments for the three months ended March 31, 2017 . During the three months ended March 31, 2016, our largest outstanding balance of reverse repurchase commitments was $265.0 million , which was open for one day. For the 10 days we invested in these transactions, the average daily balance of reverse repurchase commitments was $ 160.7 million . To the extent our repurchase and reverse repurchase transactions were with the same counterparty and subject to an enforceable master netting arrangement, we could elect to offset these transactions. Consistent with past practice, we have elected not to offset these transactions and therefore report these transactions on a gross basis on our balance sheets. Hybrid Securities Included in our fixed-maturity and equity securities are hybrid securities, which are reported at fair value: March 31, December 31, (millions) 2017 2016 Fixed maturities: Corporate debt securities $ 44.5 $ 39.2 $ 40.1 Residential mortgage-backed securities 186.6 174.4 170.5 Other asset-backed securities 8.3 10.6 8.9 Total fixed maturities 239.4 224.2 219.5 Equity securities: Nonredeemable preferred stocks 0 49.5 0 Total hybrid securities $ 239.4 $ 273.7 $ 219.5 Certain corporate debt securities are accounted for as hybrid securities since they were acquired at a premium and contain a change-in-control put option (derivative) that permits the investor, at its sole option if and when a change in control is triggered, to put the security back to the issuer at a 1% premium to par. Due to this change-in-control put option and the substantial market premium paid to acquire these securities, there is the potential that the election to put, upon the change in control, would result in an acceleration of the recognition of the remaining premium paid on these securities in our results of operations. This would result in a loss of $3.1 million as of March 31, 2017 , if all of the bonds experienced a simultaneous change in control and we elected to exercise all of our put options. The put feature limits the potential loss in value that could be experienced in the event a corporate action occurs that results in a change in control that materially diminishes the credit quality of the issuer. We are under no obligation to exercise the put option we hold if a change in control occurs. The residential mortgage-backed securities accounted for as hybrid securities are obligations of the issuer with payments of principal based on the performance of a reference pool of loans. This embedded derivative results in the securities incorporating the risk of default from both the issuer and the related loan pool. The other asset-backed security in the table above represents one hybrid security that was acquired at a deep discount to par due to a failing auction, and contains a put option that allows the investor to put that security back to the auction at par if the auction is restored. This embedded derivative had the potential to more than double our initial investment yield at acquisition. During 2016, we sold the nonredeemable preferred stocks referred to in the table above. These securities were perpetual preferred stocks with fixed-rate coupons that have call features, whereby the change in value of the call features is a component of the overall change in value of the preferred stocks. Fixed Maturities The composition of fixed maturities by maturity at March 31, 2017 , was: (millions) Cost Fair Value Less than one year $ 4,370.5 $ 4,382.3 One to five years 9,988.3 9,995.8 Five to ten years 3,030.3 3,030.0 Ten years or greater 55.7 66.2 Total $ 17,444.8 $ 17,474.3 Asset-backed securities are classified in the maturity distribution table based upon their projected cash flows. All other securities which do not have a single maturity date are reported based upon expected average maturity. Contractual maturities may differ from expected maturities because the issuers of the securities may have the right to call or prepay obligations. Gross Unrealized Losses As of March 31, 2017 , we had $90.3 million of gross unrealized losses in our fixed-income securities (i.e., fixed-maturity securities, nonredeemable preferred stocks, and short-term investments) and $4.8 million in our common equities. We currently do not intend to sell the fixed-income securities and determined that it is more likely that we will not be required to sell these securities for the period of time necessary to recover their cost bases. A review of our fixed-income securities indicated that the issuers were current with respect to their interest obligations and that there was no evidence of any deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. For common equities, 96% of our common stock portfolio was indexed to the Russell 1000; as such, this portfolio may contain securities in a loss position for an extended period of time, subject to possible write-downs, as described below. We may retain these securities as long as the portfolio and index correlation remain similar. To the extent there is issuer-specific deterioration, we may write down the securities of that issuer. The remaining 4% of our common stocks were part of a managed equity strategy selected and administered by an external investment advisor. If our review of loss position securities were to indicate there was a fundamental, or market, impairment on these securities that was determined to be other-than-temporary, we would recognize a write-down in accordance with our stated policy. The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses March 31, 2017 Fixed maturities: U.S. government obligations 34 $ 2,747.0 $ (20.6 ) 34 $ 2,747.0 $ (20.6 ) 0 $ 0 $ 0 State and local government obligations 340 986.3 (10.7 ) 310 906.5 (9.5 ) 30 79.8 (1.2 ) Corporate debt securities 147 2,494.7 (15.0 ) 139 2,415.1 (14.9 ) 8 79.6 (0.1 ) Residential mortgage-backed securities 182 880.9 (11.0 ) 82 264.4 (1.1 ) 100 616.5 (9.9 ) Agency residential pass-through obligations 65 35.4 (0.7 ) 56 32.7 (0.6 ) 9 2.7 (0.1 ) Commercial mortgage-backed securities 92 1,131.9 (16.3 ) 73 985.7 (14.2 ) 19 146.2 (2.1 ) Other asset-backed securities 118 1,601.6 (3.1 ) 112 1,521.4 (2.8 ) 6 80.2 (0.3 ) Redeemable preferred stocks 1 11.4 (1.1 ) 0 0 0 1 11.4 (1.1 ) Total fixed maturities 979 9,889.2 (78.5 ) 806 8,872.8 (63.7 ) 173 1,016.4 (14.8 ) Equity securities: Nonredeemable preferred stocks 9 197.0 (11.8 ) 5 78.5 (0.7 ) 4 118.5 (11.1 ) Common equities 68 49.2 (4.8 ) 61 48.3 (4.8 ) 7 0.9 0 Total equity securities 77 246.2 (16.6 ) 66 126.8 (5.5 ) 11 119.4 (11.1 ) Total portfolio 1,056 $ 10,135.4 $ (95.1 ) 872 $ 8,999.6 $ (69.2 ) 184 $ 1,135.8 $ (25.9 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses March 31, 2016 Fixed maturities: U.S. government obligations 5 $ 74.9 $ 0 5 $ 74.9 $ 0 0 $ 0 $ 0 State and local government obligations 112 267.8 (1.2 ) 63 132.8 (0.3 ) 49 135.0 (0.9 ) Corporate debt securities 60 672.3 (6.2 ) 21 169.2 (0.4 ) 39 503.1 (5.8 ) Residential mortgage-backed securities 157 1,345.1 (28.5 ) 52 426.9 (3.9 ) 105 918.2 (24.6 ) Agency residential pass-through obligations 25 9.1 (0.1 ) 3 0.4 0 22 8.7 (0.1 ) Commercial mortgage-backed securities 130 1,185.4 (18.9 ) 49 395.4 (2.2 ) 81 790.0 (16.7 ) Other asset-backed securities 86 1,175.2 (4.2 ) 70 999.3 (3.2 ) 16 175.9 (1.0 ) Redeemable preferred stocks 9 198.8 (48.9 ) 4 88.0 (16.1 ) 5 110.8 (32.8 ) Total fixed maturities 584 4,928.6 (108.0 ) 267 2,286.9 (26.1 ) 317 2,641.7 (81.9 ) Equity securities: Nonredeemable preferred stocks 13 344.4 (24.4 ) 4 79.0 (2.6 ) 9 265.4 (21.8 ) Common equities 73 87.4 (5.7 ) 67 84.3 (5.5 ) 6 3.1 (0.2 ) Total equity securities 86 431.8 (30.1 ) 71 163.3 (8.1 ) 15 268.5 (22.0 ) Total portfolio 670 $ 5,360.4 $ (138.1 ) 338 $ 2,450.2 $ (34.2 ) 332 $ 2,910.2 $ (103.9 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses December 31, 2016 Fixed maturities: U.S. government obligations 30 $ 2,774.0 $ (29.1 ) 30 $ 2,774.0 $ (29.1 ) 0 $ 0 $ 0 State and local government obligations 618 1,497.9 (20.7 ) 584 1,404.3 (19.6 ) 34 93.6 (1.1 ) Corporate debt securities 184 2,615.1 (24.3 ) 175 2,559.9 (24.0 ) 9 55.2 (0.3 ) Residential mortgage-backed securities 178 917.7 (15.0 ) 69 175.8 (1.1 ) 109 741.9 (13.9 ) Agency residential pass-through obligations 55 36.0 (0.6 ) 48 33.9 (0.6 ) 7 2.1 0 Commercial mortgage-backed securities 111 1,347.3 (25.5 ) 85 1,061.2 (22.9 ) 26 286.1 (2.6 ) Other asset-backed securities 103 1,605.2 (4.4 ) 89 1,423.3 (3.9 ) 14 181.9 (0.5 ) Redeemable preferred stocks 2 31.0 (2.0 ) 0 0 0 2 31.0 (2.0 ) Total fixed maturities 1,281 10,824.2 (121.6 ) 1,080 9,432.4 (101.2 ) 201 1,391.8 (20.4 ) Equity securities: Nonredeemable preferred stocks 13 329.6 (16.1 ) 8 175.2 (3.8 ) 5 154.4 (12.3 ) Common equities 75 22.1 (2.1 ) 69 19.7 (1.7 ) 6 2.4 (0.4 ) Total equity securities 88 351.7 (18.2 ) 77 194.9 (5.5 ) 11 156.8 (12.7 ) Total portfolio 1,369 $ 11,175.9 $ (139.8 ) 1,157 $ 9,627.3 $ (106.7 ) 212 $ 1,548.6 $ (33.1 ) Since March 31, 2016 , the number of securities in our fixed-maturity portfolio with unrealized losses increased, primarily the result of rising interest rates during the latter part of 2016 . A decline in credit spreads for the first three months of 2017 resulted in a decrease in the number of fixed-maturity securities with unrealized losses since December 31, 2016 . We had no material decreases in valuation as a result of credit rating downgrades on our fixed-maturity securities. All of the fixed-maturity securities in an unrealized loss position at March 31, 2017 in the table above are current with respect to required principal and interest payments. Since December 31, 2016 , our nonredeemable preferred stocks with unrealized losses decreased to nine securities, averaging approximately 6% of their total cost. The decrease in the number of securities is the result of valuation increases in the portfolio. We reviewed these securities and concluded that the unrealized losses are market-related adjustments to the values, which we determined not to be other-than-temporary; we expect to recover our initial investments on these securities. The number of issuers with unrealized losses in our common stock portfolio decreased during the first three months of 2017. A review of the securities in a loss position did not uncover fundamental issues with the issuers that would indicate other-than-temporary impairments existed. Additionally, market expectations for recovery in the next 12 months would put the fair values at or above our current book values. Lastly, we determined, as of the balance sheet date, that it was not likely these securities would be sold prior to that recovery. Other-Than-Temporary Impairment (OTTI) The following table shows the total non-credit portion of the OTTI recorded in accumulated other comprehensive income, reflecting the original non-credit loss at the time the credit impairment was determined (i.e., unadjusted for valuation changes subsequent to the original write-down): March 31, December 31, (millions) 2017 2016 Fixed maturities: Residential mortgage-backed securities $ (43.3 ) $ (43.3 ) $ (43.3 ) Commercial mortgage-backed securities (0.5 ) (0.6 ) (0.6 ) Total fixed maturities $ (43.8 ) $ (43.9 ) $ (43.9 ) The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended March 31, 2017 and 2016 , for which a portion of the OTTI losses were also recognized in accumulated other comprehensive income at the time the credit impairments were determined and recognized: Three Months Ended March 31, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Reductions for securities sold/matured 0 (0.3 ) (0.3 ) Change in recoveries of future cash flows expected to be collected 1 0.3 0 0.3 Balance at March 31, 2017 $ 11.4 $ 0.1 $ 11.5 Three Months Ended March 31, 2016 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2015 $ 12.4 $ 0.4 $ 12.8 Reductions for securities sold/matured 0 0 0 Change in recoveries of future cash flows expected to be collected 1 (0.3 ) 0 (0.3 ) Balance at March 31, 2016 $ 12.1 $ 0.4 $ 12.5 1 Reflects the current period change in the expected recovery of prior impairments that will be accreted into income over the remaining life of the security. Although we determined it is more likely that we will not be required to sell the securities prior to the recovery of their respective cost bases (which could be maturity), we are required to measure the amount of potential credit losses on the securities that were in an unrealized loss position. In that process, we considered a number of factors and inputs related to the individual securities. The methodology and significant inputs used to measure the amount of credit losses in our portfolio included: current performance indicators on the business model or underlying assets (e.g., delinquency rates, foreclosure rates, and default rates); credit support (via current levels of subordination); historical credit ratings; and updated cash flow expectations based upon these performance indicators. In order to determine the amount of credit loss, if any, the net present value of the cash flows expected (i.e., expected recovery value) was calculated using the current book yield for each security, and was compared to its current amortized value. In the event that the net present value was below the amortized value, a credit loss would be deemed to exist, and the security would be written down. We did not have any credit impairment write-downs for the three months ended March 31, 2017 or 2016 . Realized Gains (Losses) The components of net realized gains (losses) for the three months ended March 31, were: Three Months (millions) 2017 2016 Gross realized gains on security sales Fixed maturities: U.S. government obligations $ 0.4 $ 14.3 State and local government obligations 0.5 10.9 Corporate and other debt securities 4.1 12.5 Residential mortgage-backed securities 0.1 0.9 Commercial mortgage-backed securities 1.2 3.6 Other asset-backed securities 0.3 0 Redeemable preferred stocks 0.3 0 Total fixed maturities 6.9 42.2 Equity securities: Nonredeemable preferred stocks 45.4 1.6 Common equities 7.4 9.5 Subtotal gross realized gains on security sales 59.7 53.3 Gross realized losses on security sales Fixed maturities: U.S. government obligations (3.2 ) (0.4 ) State and local government obligations (0.1 ) (0.1 ) Corporate and other debt securities (0.9 ) (0.4 ) Commercial mortgage-backed securities (2.4 ) (2.7 ) Total fixed maturities (6.6 ) (3.6 ) Equity securities: Nonredeemable preferred stocks (1.2 ) (1.0 ) Common equities (0.1 ) (4.9 ) Subtotal gross realized losses on security sales (7.9 ) (9.5 ) Net realized gains (losses) on security sales Fixed maturities: U.S. government obligations (2.8 ) 13.9 State and local government obligations 0.4 10.8 Corporate and other debt securities 3.2 12.1 Residential mortgage-backed securities 0.1 0.9 Commercial mortgage-backed securities (1.2 ) 0.9 Other asset-backed securities 0.3 0 Redeemable preferred stocks 0.3 0 Total fixed maturities 0.3 38.6 Equity securities: Nonredeemable preferred stocks 44.2 0.6 Common equities 7.3 4.6 Subtotal net realized gains (losses) on security sales 51.8 43.8 Other-than-temporary impairment losses Equity securities: Common equities (1.0 ) 0 Subtotal other-than-temporary impairment losses (1.0 ) 0 Other gains (losses) Hybrid securities 0.8 (0.7 ) Derivative instruments 0 (25.7 ) Litigation settlements 0.3 0 Subtotal other gains (losses) 1.1 (26.4 ) Total net realized gains (losses) on securities $ 51.9 $ 17.4 Gross realized gains and losses were predominantly the result of sales transactions in our fixed-income portfolio related to movements in credit spreads and interest rates and sales from our equity portfolios. Our preferred stock portfolio reflects a large realized gain due primarily to one issue called by the issuer at par. This security was held at a deep discount due to previous other-than-temporary impairment write-downs taken during the market crisis of 2008. Subsequent to the write-down, the security experienced significant recovery and was trading near its anticipated call price. Upon call, we recognized the difference between the consideration received and our book value as a realized gain. In addition, gains and losses reflect recoveries from litigation settlements related to investments and holding period valuation changes on hybrids and derivatives. Also included are write-downs for securities determined to be other-than-temporarily impaired. Net Investment Income The components of net investment income for the three months ended March 31, were: Three Months (millions) 2017 2016 Fixed maturities: U.S. government obligations $ 12.7 $ 4.8 State and local government obligations 13.3 13.5 Foreign government obligations 0.1 0.1 Corporate debt securities 29.6 28.2 Residential mortgage-backed securities 9.4 12.2 Agency residential pass-through obligations 0.2 0.4 Commercial mortgage-backed securities 18.8 20.9 Other asset-backed securities 9.9 6.0 Redeemable preferred stocks 3.4 3.8 Total fixed maturities 97.4 89.9 Equity securities: Nonredeemable preferred stocks 11.5 11.8 Common equities 13.3 14.5 Short-term investments 7.0 2.6 Investment income 129.2 118.8 Investment expenses (5.6 ) (4.8 ) Net investment income $ 123.6 $ 114.0 The amount of investment income (interest and dividends) we recognize varies from year to year based on the average assets held during the year and the book yields of the securities in our portfolio. Trading Securities At March 31, 2017 and 2016 , and December 31, 2016 , we did not hold any trading securities and did not have any net realized gains (losses) on trading securities for the three months ended March 31, 2017 and 2016 . Derivative Instruments The following table shows the status of our derivative instruments at March 31, 2017 and 2016 , and December 31, 2016 , and for the three months ended March 31, 2017 and 2016 : (millions) Balance Sheet 2 Comprehensive Income Statement Assets (Liabilities) Pretax Net Realized Gains (Losses) Notional Value 1 Three Months Ended March 31, Dec. 31, March 31, Dec. 31, March 31, Derivatives designated as: 2017 2016 2016 Purpose Classification 2017 2016 2016 2017 2016 Hedging instrument Open: Interest rate swap $ 850 $ 0 $ 0 Manage Other assets $ 4.0 $ 0 $ 0 $ 0 $ 0 Closed: Ineffective cash flow hedge 31 0 370 Manage interest rate risk NA 0 0 0 0 0 Non-hedging instruments Liabilities: Interest rate swaps 0 565 0 Manage portfolio duration Other liabilities 0 (18.7 ) 0 0 (24.0 ) Closed: Interest rate swaps 0 185 750 Manage portfolio duration NA 0 0 0 0 (1.9 ) U.S. Treasury Note futures 0 55 135 Manage NA 0 0 0 0 0.2 Total NA NA NA $ 4.0 $ (18.7 ) $ 0 $ 0 $ (25.7 ) NA= Not applicable 1 The amounts represent the value held at quarter and year end for open positions and the maximum amount held during the period for closed positions. 2 To the extent we hold both derivative assets and liabilities with the same counterparty that are subject to an enforceable master netting arrangement, we reported them on a gross basis on our balance sheets, consistent with our historical presentation. CASH FLOW HEDGES During the first quarter 2017 , we entered into a forecasted debt issuance hedge, against a possible rise in interest rates, with a 30 -year interest rate swap for which we were paying a fixed rate and receiving a variable rate, in anticipation of a debt issuance. The $4.0 million pretax unrealized gain was recorded as part of accumulated other comprehensive income. At March 31, 2017 , the balance of our margin collateral at the clearinghouse was $136.1 million . During 2016, we entered into a $350 million forecasted transaction to hedge against a possible rise in interest rates in anticipation of a debt offering under which we issued $500 million of 2.45% Senior Notes due 2027. The remaining portion of our ineffective cash flow hedge resulted from the repurchase of a portion of our 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 during 2017 and 2016 , and we reclassified the unrealized gain on forecasted transactions to net realized gains on securities. The portion repurchased in 2017 resulted in an immaterial gain. See Note 4 – Debt for further discussion. INTEREST RATE SWAPS and U.S. TREASURY FUTURES We use interest rate swaps and treasury futures contracts from time to time to manage the fixed-income portfolio duration. We did not hold any interest rate swap positions at March 31, 2017 or December 31, 2016 . At March 31, 2016 , we held interest rate swap positions for which we were paying a fixed rate and receiving a variable rate, effectively shortening the duration of our fixed-income portfolio. As of March 31, 2016 , the balance of the cash collateral that we delivered to the applicable counterparties on the then open interest rate swaps was $20.6 million . We did not open any U.S. treasury futures during 2017 . During 2016 , we opened and closed treasury futures; no positions were outstanding at the end of the first quarter or year end.

Fair Value

Fair Value	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value	Fair Value — We have categorized our financial instruments, based on the degree of subjectivity inherent in the method by which they are valued, into a fair value hierarchy of three levels, as follows: • Level 1 : Inputs are unadjusted quoted prices in active markets for identical instruments at the measurement date (e.g., U.S. government obligations, active exchange-traded equity securities, and certain short-term securities). • Level 2 : Inputs (other than quoted prices included within Level 1) that are observable for the instrument either directly or indirectly (e.g., certain corporate and municipal bonds and certain preferred stocks). This includes: (i) quoted prices for similar instruments in active markets, (ii) quoted prices for identical or similar instruments in markets that are not active, (iii) inputs other than quoted prices that are observable for the instruments, and (iv) inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3 : Inputs that are unobservable. Unobservable inputs reflect our subjective evaluation about the assumptions market participants would use in pricing the financial instrument (e.g., certain structured securities and privately held investments). Determining the fair value of the investment portfolio is the responsibility of management. As part of the responsibility, we evaluate whether a market is distressed or inactive in determining the fair value for our portfolio. We review certain market level inputs to evaluate whether sufficient activity, volume, and new issuances exist to create an active market. Based on this evaluation, we concluded that there was sufficient activity related to the sectors and securities for which we obtained valuations. The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2017 Fixed maturities: U.S. government obligations $ 3,325.8 $ 0 $ 0 $ 3,325.8 $ 3,345.5 State and local government obligations 0 2,488.4 0 2,488.4 2,475.8 Foreign government obligations 22.9 0 0 22.9 22.9 Corporate debt securities 0 5,208.0 0 5,208.0 5,199.6 Subtotal 3,348.7 7,696.4 0 11,045.1 11,043.8 Asset-backed securities: Residential mortgage-backed 0 1,409.6 0 1,409.6 1,393.9 Agency residential pass-through obligations 0 38.5 0 38.5 39.1 Commercial mortgage-backed 0 2,213.5 0 2,213.5 2,216.1 Other asset-backed 0 2,564.9 0 2,564.9 2,562.4 Subtotal asset-backed securities 0 6,226.5 0 6,226.5 6,211.5 Redeemable preferred stocks: Financials 0 63.2 0 63.2 60.4 Utilities 0 31.4 0 31.4 30.6 Industrials 0 108.1 0 108.1 98.5 Subtotal redeemable preferred stocks 0 202.7 0 202.7 189.5 Total fixed maturities 3,348.7 14,125.6 0 17,474.3 17,444.8 Equity securities: Nonredeemable preferred stocks: Financials 131.1 678.7 0 809.8 710.3 Subtotal nonredeemable preferred stocks 131.1 678.7 0 809.8 710.3 Common equities: Common stocks 2,977.8 0 0 2,977.8 1,451.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,977.8 0 0.3 2,978.1 1,451.5 Total fixed maturities and equity securities 6,457.6 14,804.3 0.3 21,262.2 19,606.6 Short-term investments 2,226.7 814.1 0 3,040.8 3,040.8 Total portfolio $ 8,684.3 $ 15,618.4 $ 0.3 $ 24,303.0 $ 22,647.4 Debt $ 0 $ 3,198.3 $ 121.1 $ 3,319.4 $ 3,111.7 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2016 Fixed maturities: U.S. government obligations $ 1,388.0 $ 0 $ 0 $ 1,388.0 $ 1,375.8 State and local government obligations 0 2,508.2 0 2,508.2 2,452.0 Foreign government obligations 21.9 0 0 21.9 21.9 Corporate debt securities 0 3,840.0 0 3,840.0 3,795.2 Subtotal 1,409.9 6,348.2 0 7,758.1 7,644.9 Asset-backed securities: Residential mortgage-backed 0 1,801.5 0 1,801.5 1,810.9 Agency residential pass-through obligations 0 46.6 0 46.6 46.5 Commercial mortgage-backed 0 2,416.7 9.5 2,426.2 2,418.7 Other asset-backed 0 1,703.7 0 1,703.7 1,706.3 Subtotal asset-backed securities 0 5,968.5 9.5 5,978.0 5,982.4 Redeemable preferred stocks: Financials 0 91.7 0 91.7 81.8 Utilities 0 51.6 0 51.6 65.1 Industrials 0 87.0 0 87.0 118.1 Subtotal redeemable preferred stocks 0 230.3 0 230.3 265.0 Total fixed maturities 1,409.9 12,547.0 9.5 13,966.4 13,892.3 Equity securities: Nonredeemable preferred stocks: Financials 153.8 678.4 0 832.2 731.9 Subtotal nonredeemable preferred stocks 153.8 678.4 0 832.2 731.9 Common equities: Common stocks 2,704.7 0 0 2,704.7 1,523.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,704.7 0 0.3 2,705.0 1,524.0 Total fixed maturities and equity securities 4,268.4 13,225.4 9.8 17,503.6 16,148.2 Short-term investments 3,730.0 96.9 0 3,826.9 3,826.9 Total portfolio $ 7,998.4 $ 13,322.3 $ 9.8 $ 21,330.5 $ 19,975.1 Debt $ 0 $ 2,789.2 $ 158.1 $ 2,947.3 $ 2,701.6 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2016 Fixed maturities: U.S. government obligations $ 2,870.1 $ 0 $ 0 $ 2,870.1 $ 2,899.2 State and local government obligations 0 2,502.6 0 2,502.6 2,509.5 Foreign government obligations 24.5 0 0 24.5 24.5 Corporate debt securities 0 4,550.9 0 4,550.9 4,557.8 Subtotal 2,894.6 7,053.5 0 9,948.1 9,991.0 Asset-backed securities: Residential mortgage-backed 0 1,458.7 0 1,458.7 1,448.5 Agency residential pass-through obligations 0 40.6 0 40.6 41.2 Commercial mortgage-backed 0 2,253.1 0.3 2,253.4 2,266.9 Other asset-backed 0 2,351.1 0 2,351.1 2,350.7 Subtotal asset-backed securities 0 6,103.5 0.3 6,103.8 6,107.3 Redeemable preferred stocks: Financials 0 59.5 0 59.5 59.8 Utilities 0 30.9 0 30.9 30.5 Industrials 0 101.5 0 101.5 98.5 Subtotal redeemable preferred stocks 0 191.9 0 191.9 188.8 Total fixed maturities 2,894.6 13,348.9 0.3 16,243.8 16,287.1 Equity securities: Nonredeemable preferred stocks: Financials 138.1 715.4 0 853.5 734.2 Subtotal nonredeemable preferred stocks 138.1 715.4 0 853.5 734.2 Common equities: Common stocks 2,812.0 0 0 2,812.0 1,437.1 Other risk investments 0 0 0.4 0.4 0.4 Subtotal common equities 2,812.0 0 0.4 2,812.4 1,437.5 Total fixed maturities and equity securities 5,844.7 14,064.3 0.7 19,909.7 18,458.8 Short-term investments 3,009.3 563.6 0 3,572.9 3,572.9 Total portfolio $ 8,854.0 $ 14,627.9 $ 0.7 $ 23,482.6 $ 22,031.7 Debt $ 0 $ 3,188.5 $ 127.3 $ 3,315.8 $ 3,148.2 Our portfolio valuations, excluding short-term investments, classified as either Level 1 or Level 2 in the above tables are priced exclusively by external sources, including: pricing vendors, dealers/market makers, and exchange-quoted prices. We did not have any transfers between Level 1 and Level 2 during 2017 or 2016 . We recognize transfers between levels at the end of the reporting period. Our short-term security holdings classified as Level 1 are highly liquid, actively marketed, and have a very short duration, primarily 30 days or less to redemption. These securities are held at their original cost, adjusted for any accretion of discount, since that value very closely approximates what an active market participant would be willing to pay for such securities. The remainder of our short-term securities are classified as Level 2 and are not priced externally since these securities continually trade at par value. These securities are classified as Level 2 since they are primarily longer-dated auction securities issued by municipalities that contain a redemption put feature back to the auction pool with a redemption period typically less than seven days. The auction pool is created by a liquidity provider and if the auction is not available at the end of the seven days, we have the right to put the security back to the issuer at par. At March 31, 2017 , vendor-quoted prices represented 54% of our Level 1 classifications (excluding short-term investments), compared to 37% and 52% at March 31, 2016 and December 31, 2016 , respectively. The securities quoted by vendors in Level 1 primarily represent our holdings in U.S. Treasury Notes, which are frequently traded, and the quotes are considered similar to exchange-traded quotes. The balance of our Level 1 pricing comes from quotes obtained directly from trades made on active exchanges. At March 31, 2017 and 2016 , and December 31, 2016 , vendor-quoted prices comprised 99% , 98% , and 99% , respectively, of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented 1% , 2% , and 1% , respectively. In our process for selecting a source (e.g., dealer, pricing service) to provide pricing for securities in our portfolio, we reviewed documentation from the sources that detailed the pricing techniques and methodologies used by these sources and determined if their policies adequately considered market activity, either based on specific transactions for the particular security type or based on modeling of securities with similar credit quality, duration, yield, and structure that were recently transacted. Once a source is chosen, we continue to monitor any changes or modifications to their processes by reviewing their documentation on internal controls for pricing and market reviews. We review quality control measures of our sources as they become available to determine if any significant changes have occurred from period to period that might indicate issues or concerns regarding their evaluation or market coverage. As part of our pricing procedures, we obtain quotes from more than one source to help us fully evaluate the market price of securities. However, our internal pricing policy is to use a consistent source for individual securities in order to maintain the integrity of our valuation process. Quotes obtained from the sources are not considered binding offers to transact. Under our policy, when a review of the valuation received from our selected source appears to be outside of what is considered market level activity (which is defined as trading at spreads or yields significantly different than those of comparable securities or outside the general sector level movement without a reasonable explanation), we may use an alternate source’s price. To the extent we determine that it may be prudent to substitute one source’s price for another, we will contact the initial source to obtain an understanding of the factors that may be contributing to the significant price variance, which often leads the source to adjust their pricing input data for future pricing. To allow us to determine if our initial source is providing a price that is outside of a reasonable range, we review our portfolio pricing on a weekly basis. When necessary, we challenge prices from our sources when a price provided does not match our expectations based on our evaluation of market trends and activity. Initially, we perform a review of our portfolio by sector to identify securities whose prices appear outside of a reasonable range. We then perform a more detailed review of fair values for securities disclosed as Level 2. We review dealer bids and quotes for these and/or similar securities to determine the market level context for our valuations. We then evaluate inputs relevant for each class of securities disclosed in the preceding hierarchy tables. For our structured debt securities, including commercial, residential, and asset-backed securities, we evaluate available market-related data for these and similar securities related to collateral, delinquencies, and defaults for historical trends and reasonably estimable projections, as well as historical prepayment rates and current prepayment assumptions and cash flow estimates. We further stratify each class of our structured debt securities into more finite sectors (e.g., planned amortization class, first pay, second pay, senior, subordinated, etc.) and use duration, credit quality, and coupon to determine if the fair value is appropriate. For our corporate debt and preferred stock (redeemable and nonredeemable) portfolios, as well as the notes and debentures issued by The Progressive Corporation (see Note 4 – Debt ), we review securities by duration, coupon, and credit quality, as well as changes in interest rate and credit spread movements within that stratification. The review also includes recent trades, including: volume traded at various levels that establish a market, issuer specific fundamentals, and industry specific economic news as it comes to light. For our municipal securities (e.g., general obligations, revenue, and housing), we stratify the portfolio to evaluate securities by type, coupon, credit quality, and duration to review price changes relative to credit spread and interest rate changes. Additionally, we look to economic data as it relates to geographic location as an indication of price-to-call or maturity predictors. For municipal housing securities, we look to changes in cash flow projections, both historical and reasonably estimable projections, to understand yield changes and their effect on valuation. Lastly, for our short-term securities, we look at acquisition price relative to the coupon or yield. Since our short-term securities are typically 90 days or less to maturity, with the majority listed in Level 2 being seven days or less to redemption, we believe that acquisition price is the best estimate of fair value. We also review data assumptions as supplied by our sources to determine if that data is relevant to current market conditions. In addition, we independently review each sector for transaction volumes, new issuances, and changes in spreads, as well as the overall movement of interest rates along the yield curve to determine if sufficient activity and liquidity exists to provide a credible source for our market valuations. During each valuation period, we create internal estimations of portfolio valuation (performance returns), based on current market-related activity (i.e., interest rate and credit spread movements and other credit-related factors) within each major sector of our portfolio. We compare our internally generated portfolio results with those generated based on quotes we received externally and research material valuation differences. We compare our results to index returns for each major sector adjusting for duration and credit quality differences to better understand our portfolio’s results. Additionally, we review on a monthly basis our external sales transactions and compare the actual final market sales price to a previous market valuation price. This review provides us further validation that our pricing sources are providing market level prices, since we are able to explain significant price changes (i.e., greater than 2%) as known events occur in the marketplace and affect a particular security’s price at sale. This analysis provides us with additional comfort regarding the source’s process, the quality of its review, and its willingness to improve its analysis based on feedback from clients. We believe this effort helps ensure that we are reporting the most representative fair values for our securities. Except as described below, our Level 3 securities are also priced externally; however, due to several factors (e.g., nature of the securities, level of activity, and lack of similar securities trading to obtain observable market level inputs), these valuations are more subjective in nature. Certain private equity investments and fixed-income investments included in the Level 3 category are valued using external pricing supplemented by internal review and analysis. After all the valuations are received and our review is complete, if the inputs used by vendors are determined to not contain sufficient observable market information, we will reclassify the affected security valuations to Level 3. At March 31, 2017 and 2016 , and December 31, 2016 , securities in our fixed-maturity portfolio listed as Level 3 were comprised substantially of securities that were either: (i) private placements, (ii) thinly held and/or traded securities, or (iii) non-investment-grade or non-rated securities with little liquidity. Based on these factors, it was difficult to independently verify observable market inputs that were used to generate the external valuations we received. Despite the lack of sufficient observable market information for our Level 3 securities, we believe the valuations received in conjunction with our procedures for evaluating third-party prices support the fair values reported in the financial statements. We did not hold any internally priced securities at March 31, 2017 and 2016 , or December 31, 2016 . We review the prices from our external sources for reasonableness using internally developed assumptions to derive prices for the securities, which are then compared to the prices we received. During 2017 or 2016 , there were no material assets or liabilities measured at fair value on a nonrecurring basis. Based on our review, all prices received from external sources remained unadjusted. The following tables provide a summary of changes in fair value associated with Level 3 assets for the three months ended March 31, 2017 and 2016 : Level 3 Fair Value Three Months Ended March 31, 2017 (millions) Fair Value at December 31, 2016 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain) Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at March 31, 2017 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 $ (0.3 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 Total fixed maturities 0.3 (0.3 ) 0 0 0 0 0 0 Equity securities: Common equities: Other risk investments 0.4 (0.1 ) 0 0 0 0 0 0.3 Total Level 3 securities $ 0.7 $ (0.4 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0.3 Level 3 Fair Value Three Months Ended March 31, 2016 (millions) Fair Value at December 31, 2015 Calls/ Purchases Sales Net Realized (Gain) Loss on Sales Change in Net Fair Value at March 31, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.9 $ (0.3 ) $ 0 $ 0 $ 0 $ (0.1 ) $ 0 $ 9.5 Total fixed maturities 9.9 (0.3 ) 0 0 0 (0.1 ) 0 9.5 Equity securities: Common equities: Other risk investments 0.3 0 0 0 0 0 0 0.3 Total Level 3 securities $ 10.2 $ (0.3 ) $ 0 $ 0 $ 0 $ (0.1 ) $ 0 $ 9.8 The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at March 31, 2017 and 2016 , and December 31, 2016 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2017 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0 NA NA NA Subtotal Level 3 securities 0 Pricing exemption securities 1 0.3 Total Level 3 securities $ 0.3 NA= Not applicable, since we did not hold any commercial mortgage-backed Level 3 securities at March 31, 2017 . 1 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2016 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.5 External vendor Prepayment rate 1 0 % Subtotal Level 3 securities 9.5 Pricing exemption securities 2 0.3 Total Level 3 securities $ 9.8 1 Assumes that one security has 0% of the principal amount of the underlying loans that will be paid off prematurely in each year. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2016 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 External vendor Prepayment rate 1 0 % Subtotal Level 3 securities 0.3 Pricing exemption securities 2 0.4 Total Level 3 securities $ 0.7 1 Assumes that one security has 0% of the principal amount of the underlying loans that will be paid off prematurely in each year. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Due to the relative size of the Level 3 securities’ fair values compared to the total portfolio’s fair value, any changes in pricing methodology would not have a significant change in valuation that would materially impact net or comprehensive income.

Debt

Debt	3 Months Ended
	Mar. 31, 2017
Debt Disclosure [Abstract]
Debt	Debt — Debt at each of the balance sheet periods consisted of: March 31, 2017 March 31, 2016 December 31, 2016 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 498.5 $ 527.9 $ 498.2 $ 538.4 $ 498.4 $ 528.8 2.45% Senior Notes due 2027 495.8 468.6 0 0 495.8 464.6 6 5/8% Senior Notes due 2029 296.0 386.3 295.8 388.7 295.9 380.1 6.25% Senior Notes due 2032 395.2 512.1 395.0 509.7 395.2 499.0 4.35% Senior Notes due 2044 346.5 366.7 346.4 377.9 346.4 362.3 3.70% Senior Notes due 2045 395.1 380.0 395.0 388.6 395.1 372.5 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 563.5 556.7 613.1 585.9 594.1 581.2 Other debt instruments 121.1 121.1 158.1 158.1 127.3 127.3 Total $ 3,111.7 $ 3,319.4 $ 2,701.6 $ 2,947.3 $ 3,148.2 $ 3,315.8 The other debt instruments reported in the table above represent ARX indebtedness and consist of: March 31, 2017 March 31, 2016 December 31, 2016 ($ in millions) Type of debt instrument Number of Instruments Carrying Value Number of Instruments Carrying Number of Instruments Carrying Stated Maturity Date(s) Term loans 2 $ 55.9 2 $ 80.8 2 $ 62.1 December 2018 and 2019 Junior subordinated notes 1 2 41.2 2 41.2 2 41.2 June 2036 and 2037 Senior notes 4 24.0 4 24.0 4 24.0 Various 2 Surplus note 3 0 0 1 12.1 0 0 November 2021 Total $ 121.1 $ 158.1 $ 127.3 1 ARX issued junior subordinated floating rate notes to trusts established by ARX in connection with issuances of trust preferred securities by the trusts (discussed below). 2 The senior notes mature in May 2033, April 2034, December 2034, and June 2035. 3 The surplus note was debt of the subsidiary disposed of by ARX in the exchange transaction during the second quarter 2016. The Progressive Corporation Debt During the first quarter 2017 and the full year 2016, we repurchased, in the open market, $30.9 million and $19.8 million , respectively, in aggregate principal amount of our 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 (the " 6.70% Debentures"). Since the carrying value of the debt we repurchased differed from the amount paid to extinguish the debt, we recognized a gain of $0.2 million during the first quarter 2017 and $1.6 million in 2016. We did not repurchase any debt securities in the first quarter 2016. During the year ended December 31, 2016 , we issued $500 million of our 2.45% Senior Notes due 2027 in an underwritten public offering. We received proceeds, after deducting underwriter's discounts, commissions, and other issuance costs, of approximately $495.6 million . See Note 15 – Subsequent Event , for a discussion of senior notes issued in April 2017. ARX Debt (i.e., Other debt instruments) The other debt instruments were issued by ARX, prior to The Progressive Corporation acquiring a controlling interest in 2015. ARX, not The Progressive Corporation or any of its other subsidiaries, is responsible for the other debt, which includes amounts that were borrowed and contributed to the capital of ARX's insurance subsidiaries or used, or made available for use, for other business purposes. In estimating the fair values of the other debt instruments, it was determined that the fair values of these notes are equal to the carrying value, based on the current rates offered for debt of similar maturities and interest rates. Pursuant to agreements entered into by ARX relating to the trust preferred securities transactions, ARX established trusts that are entirely owned by ARX. The trusts, which are the holders of the junior subordinated notes, issued trust preferred securities to third parties. The shares in the trusts are not transferable. The trusts are considered special purpose variable interest entities for which ARX is not the primary beneficiary and, therefore, they are accounted for under the equity method of accounting and not consolidated with ARX. Our ownership interest of $1.3 million at March 31, 2017 and 2016, and December 31, 2016, in the variable interest entities is reported as a component of "other assets" on our consolidated balance sheets. The Progressive Corporation Line of Credit During 2016, we renewed the unsecured, discretionary line of credit (the "Line of Credit") with PNC Bank, National Association (PNC) in the maximum principal amount of $100 million . Subject to the terms and conditions of the Line of Credit documents, advances under the Line of Credit (if any) will bear interest at a variable rate equal to the higher of PNC's Prime Rate or the sum of the Federal Funds Open Rate plus 50 basis points. Each advance must be repaid on the 30 th day after the advance or, if earlier, on April 30, 2017, the expiration date of the Line of Credit. Prepayments are permitted without penalty. All advances under the Line of Credit are subject to PNC's discretion. We had no borrowings under the Line of Credit during the first three months of 2017 or throughout 2016.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes — At March 31, 2017 and 2016 , and December 31, 2016 , we determined that we did not need a valuation allowance on our gross deferred tax assets. Although realization of the deferred tax assets is not assured, management believes that it is more likely than not that the deferred tax assets will be realized based on our expectation that we will be able to fully utilize the deductions that are ultimately recognized for tax purposes. For the three months ended March 31, 2017 , there have been no material changes in our uncertain tax positions or effective tax rate.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	3 Months Ended
	Mar. 31, 2017
Loss and Loss Adjustment Expenses Reserves [Abstract]
Loss and Loss Adjustment Expenses Reserves	Loss and Loss Adjustment Expense Reserves — Activity in the loss and loss adjustment expense reserves is summarized as follows: March 31, (millions) 2017 2016 Balance, Beginning of period $ 11,368.0 $ 10,039.0 Less reinsurance recoverables on unpaid losses 1,801.0 1,442.7 Net balance, Beginning of period 9,567.0 8,596.3 Incurred related to: Current year 4,152.2 3,853.0 Prior years 111.2 60.4 Total incurred 4,263.4 3,913.4 Paid related to: Current year 1,933.2 1,864.5 Prior years 2,118.7 1,867.2 Total paid 4,051.9 3,731.7 Net balance, End of period 9,778.5 8,778.0 Plus reinsurance recoverables on unpaid losses 1,850.4 1,508.7 Balance, End of period $ 11,628.9 $ 10,286.7 We experienced unfavorable reserve development of $111.2 million and $60.4 million for the first quarter 2017 and 2016, respectively, which is reflected as "Incurred related to prior years" in the table above. First quarter 2017 • Approximately $100 million of the unfavorable prior year reserve development was attributable to accident year 2016. • Our vehicle businesses incurred about $124 million of unfavorable loss and loss adjustment expense (LAE) reserve development for the first quarter 2017, partially offset by the favorable loss and LAE reserve development in our Property business. • Our Agency and Direct auto businesses incurred about $68 million and $37 million , respectively, of the total unfavorable reserve development, primarily due to an increase in costs related to property damage, unfavorable personal injury protection (PIP) case reserve development, mainly in Michigan, and more late reported bodily injury claims than anticipated. • Our Commercial Lines business experienced unfavorable development primarily due to more late reported injury claims than anticipated. First quarter 2016 • Approximately $46 million of the unfavorable prior year reserve development was attributable to accident year 2015. • Nearly three-fourths of our unfavorable reserve development was in our personal auto product. Our Agency and Direct auto businesses incurred about $32 million and $11 million , respectively, of the total unfavorable reserve development. • Our Commercial Lines business incurred about $15 million of the unfavorable development. • In our personal auto and Commercial Lines businesses, we incurred unfavorable incurred but not recorded (IBNR) reserve development, primarily due to a higher severity and frequency of late reported claims than anticipated for accident year 2015, driven in part by storms in late December 2015, resulting in a greater number of claims being reported in January 2016 than anticipated. • The development during the quarter for our Property business was minimal.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information — Cash includes only bank demand deposits. We paid the following in the respective periods: Three Months Ended March 31, (millions) 2017 2016 Income taxes $ 0.7 $ 30.0 Interest 33.1 28.1 Restricted cash on our consolidated balance sheets represents cash that is restricted to pay flood claims under the National Flood Insurance Program's "Write Your Own" program, for which American Strategic Insurance and other subsidiaries of ARX (ASI) is an administrator.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2017
Segment Reporting [Abstract]
Segment Information	Segment Information — Our Personal Lines segment writes insurance for personal autos and recreational vehicles (our special lines products). Our Commercial Lines segment writes primary liability and physical damage insurance for automobiles and trucks owned and/or operated predominantly by small businesses in the business auto, for-hire transportation, contractor, for-hire specialty, tow, and for-hire livery markets. Our Property segment writes residential property insurance for homeowners, other property owners, and renters. Our other indemnity businesses manage our run-off businesses. Our service businesses provide insurance-related services, including processing Commercial Automobile Insurance Procedures/Plans (CAIP) business, and serving as an agent for homeowners, general liability, and workers’ compensation insurance through our programs with ASI and unaffiliated insurance companies. All segment revenues are generated from external customers; all intercompany transactions, including those between Progressive and ASI, are eliminated in consolidation. Following are the operating results for the respective periods: Three Months Ended March 31, 2017 2016 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 2,631.5 $ 249.7 $ 2,347.9 $ 171.4 Direct 2,523.7 175.3 2,220.7 79.5 Total Personal Lines 1 5,155.2 425.0 4,568.6 250.9 Commercial Lines 645.5 67.4 548.8 61.1 Property 2 226.0 7.8 200.0 (24.5 ) Other indemnity 0 (0.2 ) 0 (0.7 ) Total underwriting operations 6,026.7 500.0 5,317.4 286.8 Fees and other revenues 3 85.2 NA 78.9 NA Service businesses 28.5 2.6 25.0 3.4 Investments 4 181.1 175.5 136.2 131.4 Gains on extinguishment of debt 0.2 0.2 0 0 Interest expense NA (36.8 ) NA (34.2 ) Consolidated total $ 6,321.7 $ 641.5 $ 5,557.5 $ 387.4 NA = Not applicable 1 Personal auto insurance accounted for 93% and 92% of the total Personal Lines segment net premiums earned in the first quarters of 2017 and 2016 , respectively; insurance for our special lines products (e.g., motorcycles, watercraft, and RVs) accounted for the balance of the Personal Lines net premiums earned. 2 For both the three months ended March 31, 2017 and 2016, pretax profit (loss) also includes $15.5 million of amortization expense predominately associated with the acquisition of a controlling interest in ARX. 3 Pretax profit (loss) for fees and other revenues are attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expenses. Our management uses underwriting margin and combined ratio as primary measures of underwriting profitability. Underwriting profitability is calculated by subtracting losses and loss adjustment expenses, policy acquisition costs, and other underwriting expenses from the total of net premiums earned and fees and other revenues. The underwriting margin is the pretax underwriting profit (loss) expressed as a percentage of net premiums earned (i.e., revenues from underwriting operations). Combined ratio is the complement of the underwriting margin. Following are the underwriting margins and combined ratios for our underwriting operations for the respective periods: Three Months Ended March 31, 2017 2016 Under-writing Combined Ratio Under-writing Combined Ratio Personal Lines Agency 9.5 % 90.5 7.3 % 92.7 Direct 6.9 93.1 3.6 96.4 Total Personal Lines 8.2 91.8 5.5 94.5 Commercial Lines 10.4 89.6 11.1 88.9 Property 1 3.4 96.6 (12.2) 112.2 Other indemnity 2 NM NM NM NM Total underwriting operations 8.3 91.7 5.4 94.6 1 Included in the three months ended March 31, 2017 and 2016, is 6.9 points and 7.8 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX. 2 Underwriting margins and combined ratios are not meaningful (NM) for our other indemnity businesses due to the low level of premiums earned by, and the variability of loss costs in, such businesses.

Dividends

Dividends	3 Months Ended
	Mar. 31, 2017
Text Block [Abstract]
Dividends	Dividends — We maintain a policy of paying an annual variable dividend that, if declared, would be payable shortly after the close of the year. This annual variable dividend is based on a target percentage of after-tax underwriting income multiplied by a performance factor (Gainshare factor), which, beginning in 2017, is determined by reference to the Agency auto, Direct auto, special lines, Commercial Lines, and Property business units, with minor exclusions and subject to the limitations discussed below. The target percentage is determined by our Board of Directors on an annual basis and announced to shareholders and the public. In December 2016 , the Board determined the target percentage for 2017 to be 33-1/3 % of annual after-tax underwriting income, which is unchanged from the 2016 target percentage. The Gainshare factor can range from zero to two and is determined by comparing our operating performance for the specified business units for the year to certain predetermined profitability and growth objectives approved by the Compensation Committee of the Board. This Gainshare factor is also used in the annual cash bonus program currently in place for our employees (our “Gainsharing program”). On a year-to-date basis, as of March 31, 2017 , the Gainshare factor was 1.83 . Since the final factor will be determined based on our results for the full year, the final factor may vary from the current factor. Our annual dividend program will result in a variable payment to shareholders each year, subject to certain limitations. If the Gainshare factor is zero or if our comprehensive income is less than after-tax underwriting income, no dividend would be payable under our annual variable dividend policy. In addition, the ultimate decision on whether or not a dividend will be paid is in the discretion of the Board of Directors. The Board could decide to alter our policy, or not to pay the annual variable dividend, at any time prior to the declaration of the dividend for the year. Such an action by the Board could result from, among other reasons, changes in the insurance marketplace, changes in our performance or capital needs, changes in federal income tax laws, disruptions of national or international capital markets, or other events affecting our business, liquidity, or financial position. Following is a summary of our shareholder dividends that were declared in the last two years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Share Total 1 Annual – Variable December 2016 February 2017 $ 0.6808 $ 395.4 Annual – Variable December 2015 February 2016 $ 0.8882 $ 519.2 1 Based on an estimate of shares outstanding as of the record date. For the dividends declared in December 2016 and 2015, we paid $ 395.4 million and $519.0 million , respectively.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2017
Equity [Abstract]
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) — The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net Net Foreign (Income)loss attributable to NCI Balance at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Other comprehensive income (loss) before reclassifications: Investment securities 254.5 (89.2 ) 165.3 165.3 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions 4.0 (1.4 ) 2.6 0 2.6 0 0 Foreign currency translation adjustment 0.5 (0.2 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) (1.5 ) 0.6 (0.9 ) 0 0 0 (0.9 ) Total other comprehensive income (loss) before reclassifications 257.5 (90.2 ) 167.3 165.3 2.6 0.3 (0.9 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (1.0 ) 0.4 (0.6 ) (0.6 ) 0 0 0 Net realized gains (losses) on securities 51.6 (18.1 ) 33.5 33.5 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 51.1 (17.9 ) 33.2 32.9 0.3 0 0 Total other comprehensive income (loss) 206.4 (72.3 ) 134.1 132.4 2.3 0.3 (0.9 ) Balance at March 31, 2017 $ 1,645.9 $ (578.4 ) $ 1,067.5 $ 1,072.0 $ (7.1 ) $ (0.8 ) $ 3.4 Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at December 31, 2015 $ 1,234.5 $ (434.1 ) $ 800.4 $ 809.0 $ (8.2 ) $ (1.5 ) $ 1.1 Other comprehensive income (loss) before reclassifications: Investment securities 142.7 (50.3 ) 92.4 92.4 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes (0.1 ) 0.1 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.7 (0.2 ) 0.5 0 0 0.5 0 Loss attributable to noncontrolling interest (NCI) (3.3 ) 1.2 (2.1 ) 0 0 0 (2.1 ) Total other comprehensive income (loss) before reclassifications 140.0 (49.2 ) 90.8 92.4 0 0.5 (2.1 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings 0 0 0 0 0 0 0 Net realized gains (losses) on securities 35.3 (12.4 ) 22.9 22.9 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 35.8 (12.6 ) 23.2 22.9 0.3 0 0 Total other comprehensive income (loss) 104.2 (36.6 ) 67.6 69.5 (0.3 ) 0.5 (2.1 ) Balance at March 31, 2016 $ 1,338.7 $ (470.7 ) $ 868.0 $ 878.5 $ (8.5 ) $ (1.0 ) $ (1.0 ) In an effort to manage interest rate risk, we entered into forecasted transactions on each of Progressive's outstanding debt issuances. Upon issuing the debt, the gains (losses) recognized on these cash flow hedges are recorded as unrealized gains (losses) in accumulated other comprehensive income and amortized into interest expense over the term of the related debt issuance. We expect to reclassify expense of $0.4 million (pretax) into income during the next 12 months, related to net unrealized losses on forecasted transactions.

Litigation

Litigation	3 Months Ended
	Mar. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Litigation	Litigation — The Progressive Corporation and/or its insurance subsidiaries are named as defendants in various lawsuits arising out of claims made under insurance policies written by our insurance subsidiaries in the ordinary course of business. We consider all legal actions relating to such claims in establishing our loss and loss adjustment expense reserves. In addition, The Progressive Corporation and/or its insurance subsidiaries are named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the insurance subsidiaries. These cases include those alleging damages as a result of our subsidiaries’ practices in evaluating or paying medical or injury claims or benefits, including, but not limited to, personal injury protection, medical payments, and bodily injury benefits; the utilization, content, or appearance of policy documents; labor rates paid to auto body repair shops; wage and hour issues; and cases challenging other aspects of our subsidiaries’ claims or marketing practices or other business operations. Other insurance companies face many of these same issues. The nature and volume of litigation to which The Progressive Corporation is subject is similar to that which was disclosed in Note 12 - Litigation in our 2016 Annual Report to Shareholders. We plan to contest the pending lawsuits vigorously, but may pursue settlement negotiations in some cases, if appropriate. The outcomes of pending cases are uncertain at this time. We establish accruals for these lawsuits when it is probable that a loss has been or will be incurred and we can reasonably estimate potential loss exposure, which may include a range of loss. As to lawsuits for which the loss is considered neither probable or estimable, or is considered probable but not estimable, we do not establish an accrual. Nevertheless, we continue to evaluate this pending litigation to determine if any losses not deemed probable and estimable become so, at which point we would establish an accrual at our best estimate of the loss or range of loss. With respect to our pending lawsuits that are not related to claims under insurance policies, the accruals that we have established, if any, were not material at March 31, 2017 or 2016, and there have not been any material settlements during the first quarter 2017. For most of these lawsuits, we do not consider any losses to be both probable and estimable, and we are unable to estimate a meaningful range of loss, if any, at this time, due to the factors discussed in Note 12 - Litigation in our 2016 Annual Report to Shareholders. In the event that any one or more of these lawsuits results in a substantial judgment against, or settlement by, us or if our accruals (if any) prove to be inadequate by a significant amount, the resulting liability could have a material adverse effect on our consolidated financial condition, cash flows, and/or results of operations. For a further discussion on our pending litigation and related reserving policies, see Note 12 – Litigation in our 2016 Annual Report to Shareholders.

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interest	3 Months Ended
	Mar. 31, 2017
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	Redeemable Noncontrolling Interest — In connection with the April 2015 acquisition of a controlling interest in ARX, The Progressive Corporation entered into a stockholders’ agreement with the other ARX stockholders. As part of the stockholders’ agreement, the minority ARX shareholders have the right to “put” their ARX shares to Progressive in two installments, one in early 2018 and one in early 2021, and Progressive has the ability to "call" a portion of the outstanding shares shortly thereafter. If these rights are exercised in full when available, our ownership stake in ARX capital stock will exceed 80% in 2018 and will reach 100% in 2021. Since these securities are redeemable upon the occurrence of an event that is not solely within the control of Progressive, we have recorded the redeemable noncontrolling interest (NCI) as mezzanine equity on our consolidated balance sheets, which represents the minority shares at the current estimated purchase price pursuant to the put and call provisions of the stockholders' agreement. The estimated purchase price is based, in part, on the change in tangible net book value of ARX from December 31, 2014 to the balance sheet dates. In addition to these minority shares, at March 31, 2017, ARX employees hold options to purchase 22,550 ARX shares. These options and any shares issued upon exercise are subject to the stockholders' agreement, including the right to "put" these shares to Progressive, as described above. Until the options are exercised, the underlying obligation of approximately $31.9 million is not recorded as part of redeemable NCI. The changes in the components of redeemable NCI during the three months ended March 31, 2017 and 2016 , and the year ended December 31, 2016 , were: (millions) March 31, 2017 March 31, 2016 December 31, 2016 Balance, Beginning of period $ 483.7 $ 464.9 $ 464.9 Net income attributable to NCI 6.0 0.5 26.2 Other comprehensive income (loss) attributable to NCI 0.9 2.1 (3.2 ) Exercise of employee stock options 3.4 0 0 Change in redemption value of NCI 0.2 (0.1 ) (4.2 ) Balance, End of period $ 494.2 $ 467.4 $ 483.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets	3 Months Ended
	Mar. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill During the three months ended March 31, 2017 and 2016 , there were no changes to the carrying amount of goodwill. For the year ended December 31, 2016 , the carrying amount of goodwill increased $1.8 million as a result of an ARX exchange transaction in which ARX acquired the equity interest in a residential property insurance company and disposed of the equity interest in a commercial property insurance company. Goodwill recorded at March 31, 2017 was $449.4 million . No accumulated goodwill impairment losses exist. Intangible Assets The following table is a summary of the net carrying amount of other intangible assets as of March 31, 2017 and 2016 , and December 31, 2016 : (millions) March 31, 2017 March 31, 2016 December 31, 2016 Intangible assets subject to amortization $ 404.9 $ 467.0 $ 420.4 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 417.3 $ 479.4 $ 432.8 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: (millions) March 31, 2017 March 31, 2016 December 31, 2016 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 73.2 $ 183.0 $ 256.2 $ 36.6 $ 219.6 $ 256.2 $ 64.1 $ 192.1 Agency relationships 159.2 22.7 136.5 159.2 11.4 147.8 159.2 19.9 139.3 Software rights 79.1 21.5 57.6 79.1 10.8 68.3 79.1 18.8 60.3 Trade name 34.8 7.0 27.8 34.8 3.5 31.3 34.8 6.1 28.7 Total $ 529.3 $ 124.4 $ 404.9 $ 529.3 $ 62.3 $ 467.0 $ 529.3 $ 108.9 $ 420.4 Amortization expense was $15.5 million for the three months ended March 31, 2017 and 2016 .

New Accounting Standards

New Accounting Standards	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards Issued In March 2017, the Financial Accounting Standards Boards (FASB) issued an accounting standards update (ASU) related to premium amortization on purchased callable debt securities. The intent of the standard is to shorten the amortization period for certain purchased callable debt securities held at a premium. Under the ASU, the premium is required to be amortized to the earliest call date. The ASU more closely aligns interest income recorded on bonds held at a premium with the economics of the underlying instrument. The ASU, which is required to be applied on a modified retrospective basis, is effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Since we have historically used a yield-to-worst scenario for our securities that were purchased at a premium, and the first call on a premium security most often produces the lowest and most conservative yield, we do not expect this standard to have a significant impact on our financial condition, cash flows, or results of operations. Adopted On January 1, 2017, we adopted the ASU to simplify the accounting for employee share-based payment transactions. There were several provisions that could be adopted under this ASU. We did not elect to make any changes to our method of recording forfeitures and are continuing to withhold taxes at the minimum statutory tax rate. We did adopt, on a retrospective basis, to disclose the payment of cash to a taxing authority for which we withheld shares for this purpose as a financing activity. Lastly, on a prospective basis, we recognized $8.3 million of excess tax benefits as an income tax benefit in our consolidated statements of comprehensive income.

Subsequent Event

Subsequent Event	3 Months Ended
	Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event — On April 3, 2017 , we issued $850 million of 4.125% Senior Notes due 2047 (the “ 4.125% Senior Notes”). We received proceeds of about $841.1 million , after deducting underwriter’s discounts and commissions, and an estimated $1.5 million of additional expenses related to the issuance. In addition, upon issuance of the 4.125% Senior Notes, we closed a forecasted debt issuance hedge, which was entered into to hedge against a possible rise in interest rates, and recognized an $8.0 million pretax loss as part of accumulated other comprehensive income (loss); the loss will be recognized as an adjustment to interest expense and amortized over the life of the 4.125% Senior Notes.

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2017
Investments Schedule [Abstract]
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]	The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. The net holding period gains (losses) represent the amounts realized on our hybrid securities only. ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2017 Fixed maturities: U.S. government obligations $ 3,345.5 $ 0.9 $ (20.6 ) $ 0 $ 3,325.8 13.7 % State and local government obligations 2,475.8 23.3 (10.7 ) 0 2,488.4 10.2 Foreign government obligations 22.9 0 0 0 22.9 0.1 Corporate debt securities 5,199.6 23.1 (15.0 ) 0.3 5,208.0 21.4 Residential mortgage-backed securities 1,393.9 24.6 (11.0 ) 2.1 1,409.6 5.8 Agency residential pass-through obligations 39.1 0.1 (0.7 ) 0 38.5 0.2 Commercial mortgage-backed securities 2,216.1 13.7 (16.3 ) 0 2,213.5 9.1 Other asset-backed securities 2,562.4 5.4 (3.1 ) 0.2 2,564.9 10.6 Redeemable preferred stocks 189.5 14.3 (1.1 ) 0 202.7 0.8 Total fixed maturities 17,444.8 105.4 (78.5 ) 2.6 17,474.3 71.9 Equity securities: Nonredeemable preferred stocks 710.3 111.3 (11.8 ) 0 809.8 3.3 Common equities 1,451.5 1,531.4 (4.8 ) 0 2,978.1 12.3 Short-term investments 3,040.8 0 0 0 3,040.8 12.5 Total portfolio 1,2 $ 22,647.4 $ 1,748.1 $ (95.1 ) $ 2.6 $ 24,303.0 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2016 Fixed maturities: U.S. government obligations $ 1,375.8 $ 12.2 $ 0 $ 0 $ 1,388.0 6.5 % State and local government obligations 2,452.0 57.4 (1.2 ) 0 2,508.2 11.8 Foreign government obligations 21.9 0 0 0 21.9 0.1 Corporate debt securities 3,795.2 50.6 (6.2 ) 0.4 3,840.0 18.0 Residential mortgage-backed securities 1,810.9 19.5 (28.5 ) (0.4 ) 1,801.5 8.4 Agency residential pass-through obligations 46.5 0.2 (0.1 ) 0 46.6 0.2 Commercial mortgage-backed securities 2,418.7 26.4 (18.9 ) 0 2,426.2 11.4 Other asset-backed securities 1,706.3 1.3 (4.2 ) 0.3 1,703.7 8.0 Redeemable preferred stocks 265.0 14.2 (48.9 ) 0 230.3 1.1 Total fixed maturities 13,892.3 181.8 (108.0 ) 0.3 13,966.4 65.5 Equity securities: Nonredeemable preferred stocks 731.9 124.7 (24.4 ) 0 832.2 3.9 Common equities 1,524.0 1,186.7 (5.7 ) 0 2,705.0 12.7 Short-term investments 3,826.9 0 0 0 3,826.9 17.9 Total portfolio 1,2 $ 19,975.1 $ 1,493.2 $ (138.1 ) $ 0.3 $ 21,330.5 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value December 31, 2016 Fixed maturities: U.S. government obligations $ 2,899.2 $ 0 $ (29.1 ) $ 0 $ 2,870.1 12.2 % State and local government obligations 2,509.5 13.8 (20.7 ) 0 2,502.6 10.7 Foreign government obligations 24.5 0 0 0 24.5 0.1 Corporate debt securities 4,557.8 17.3 (24.3 ) 0.1 4,550.9 19.4 Residential mortgage-backed securities 1,448.5 23.7 (15.0 ) 1.5 1,458.7 6.2 Agency residential pass-through obligations 41.2 0 (0.6 ) 0 40.6 0.2 Commercial mortgage-backed securities 2,266.9 12.0 (25.5 ) 0 2,253.4 9.6 Other asset-backed securities 2,350.7 4.6 (4.4 ) 0.2 2,351.1 10.0 Redeemable preferred stocks 188.8 5.1 (2.0 ) 0 191.9 0.8 Total fixed maturities 16,287.1 76.5 (121.6 ) 1.8 16,243.8 69.2 Equity securities: Nonredeemable preferred stocks 734.2 135.4 (16.1 ) 0 853.5 3.6 Common equities 1,437.5 1,377.0 (2.1 ) 0 2,812.4 12.0 Short-term investments 3,572.9 0 0 0 3,572.9 15.2 Total portfolio 1,2 $ 22,031.7 $ 1,588.9 $ (139.8 ) $ 1.8 $ 23,482.6 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions and collateral on open derivative positions; at March 31, 2017 and 2016 , and December 31, 2016 , we had $111.0 million , $47.3 million , and $27.8 million , respectively, included in "other liabilities." In addition, at March 31, 2017, we had $136.1 million of margin collateral on an open derivative position, which was included in "other assets." 2 The total fair value of the portfolio at March 31, 2017 and 2016 , and December 31, 2016 included $0.7 billion , $0.7 billion , and $1.3 billion , respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions.
Hybrid Instruments Table [Table Text Block]	Included in our fixed-maturity and equity securities are hybrid securities, which are reported at fair value: March 31, December 31, (millions) 2017 2016 Fixed maturities: Corporate debt securities $ 44.5 $ 39.2 $ 40.1 Residential mortgage-backed securities 186.6 174.4 170.5 Other asset-backed securities 8.3 10.6 8.9 Total fixed maturities 239.4 224.2 219.5 Equity securities: Nonredeemable preferred stocks 0 49.5 0 Total hybrid securities $ 239.4 $ 273.7 $ 219.5
Investments Classified by Contractual Maturity Date [Table Text Block]	The composition of fixed maturities by maturity at March 31, 2017 , was: (millions) Cost Fair Value Less than one year $ 4,370.5 $ 4,382.3 One to five years 9,988.3 9,995.8 Five to ten years 3,030.3 3,030.0 Ten years or greater 55.7 66.2 Total $ 17,444.8 $ 17,474.3
Schedule of Unrealized Loss on Investments [Table Text Block]	The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses March 31, 2017 Fixed maturities: U.S. government obligations 34 $ 2,747.0 $ (20.6 ) 34 $ 2,747.0 $ (20.6 ) 0 $ 0 $ 0 State and local government obligations 340 986.3 (10.7 ) 310 906.5 (9.5 ) 30 79.8 (1.2 ) Corporate debt securities 147 2,494.7 (15.0 ) 139 2,415.1 (14.9 ) 8 79.6 (0.1 ) Residential mortgage-backed securities 182 880.9 (11.0 ) 82 264.4 (1.1 ) 100 616.5 (9.9 ) Agency residential pass-through obligations 65 35.4 (0.7 ) 56 32.7 (0.6 ) 9 2.7 (0.1 ) Commercial mortgage-backed securities 92 1,131.9 (16.3 ) 73 985.7 (14.2 ) 19 146.2 (2.1 ) Other asset-backed securities 118 1,601.6 (3.1 ) 112 1,521.4 (2.8 ) 6 80.2 (0.3 ) Redeemable preferred stocks 1 11.4 (1.1 ) 0 0 0 1 11.4 (1.1 ) Total fixed maturities 979 9,889.2 (78.5 ) 806 8,872.8 (63.7 ) 173 1,016.4 (14.8 ) Equity securities: Nonredeemable preferred stocks 9 197.0 (11.8 ) 5 78.5 (0.7 ) 4 118.5 (11.1 ) Common equities 68 49.2 (4.8 ) 61 48.3 (4.8 ) 7 0.9 0 Total equity securities 77 246.2 (16.6 ) 66 126.8 (5.5 ) 11 119.4 (11.1 ) Total portfolio 1,056 $ 10,135.4 $ (95.1 ) 872 $ 8,999.6 $ (69.2 ) 184 $ 1,135.8 $ (25.9 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses March 31, 2016 Fixed maturities: U.S. government obligations 5 $ 74.9 $ 0 5 $ 74.9 $ 0 0 $ 0 $ 0 State and local government obligations 112 267.8 (1.2 ) 63 132.8 (0.3 ) 49 135.0 (0.9 ) Corporate debt securities 60 672.3 (6.2 ) 21 169.2 (0.4 ) 39 503.1 (5.8 ) Residential mortgage-backed securities 157 1,345.1 (28.5 ) 52 426.9 (3.9 ) 105 918.2 (24.6 ) Agency residential pass-through obligations 25 9.1 (0.1 ) 3 0.4 0 22 8.7 (0.1 ) Commercial mortgage-backed securities 130 1,185.4 (18.9 ) 49 395.4 (2.2 ) 81 790.0 (16.7 ) Other asset-backed securities 86 1,175.2 (4.2 ) 70 999.3 (3.2 ) 16 175.9 (1.0 ) Redeemable preferred stocks 9 198.8 (48.9 ) 4 88.0 (16.1 ) 5 110.8 (32.8 ) Total fixed maturities 584 4,928.6 (108.0 ) 267 2,286.9 (26.1 ) 317 2,641.7 (81.9 ) Equity securities: Nonredeemable preferred stocks 13 344.4 (24.4 ) 4 79.0 (2.6 ) 9 265.4 (21.8 ) Common equities 73 87.4 (5.7 ) 67 84.3 (5.5 ) 6 3.1 (0.2 ) Total equity securities 86 431.8 (30.1 ) 71 163.3 (8.1 ) 15 268.5 (22.0 ) Total portfolio 670 $ 5,360.4 $ (138.1 ) 338 $ 2,450.2 $ (34.2 ) 332 $ 2,910.2 $ (103.9 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses December 31, 2016 Fixed maturities: U.S. government obligations 30 $ 2,774.0 $ (29.1 ) 30 $ 2,774.0 $ (29.1 ) 0 $ 0 $ 0 State and local government obligations 618 1,497.9 (20.7 ) 584 1,404.3 (19.6 ) 34 93.6 (1.1 ) Corporate debt securities 184 2,615.1 (24.3 ) 175 2,559.9 (24.0 ) 9 55.2 (0.3 ) Residential mortgage-backed securities 178 917.7 (15.0 ) 69 175.8 (1.1 ) 109 741.9 (13.9 ) Agency residential pass-through obligations 55 36.0 (0.6 ) 48 33.9 (0.6 ) 7 2.1 0 Commercial mortgage-backed securities 111 1,347.3 (25.5 ) 85 1,061.2 (22.9 ) 26 286.1 (2.6 ) Other asset-backed securities 103 1,605.2 (4.4 ) 89 1,423.3 (3.9 ) 14 181.9 (0.5 ) Redeemable preferred stocks 2 31.0 (2.0 ) 0 0 0 2 31.0 (2.0 ) Total fixed maturities 1,281 10,824.2 (121.6 ) 1,080 9,432.4 (101.2 ) 201 1,391.8 (20.4 ) Equity securities: Nonredeemable preferred stocks 13 329.6 (16.1 ) 8 175.2 (3.8 ) 5 154.4 (12.3 ) Common equities 75 22.1 (2.1 ) 69 19.7 (1.7 ) 6 2.4 (0.4 ) Total equity securities 88 351.7 (18.2 ) 77 194.9 (5.5 ) 11 156.8 (12.7 ) Total portfolio 1,369 $ 11,175.9 $ (139.8 ) 1,157 $ 9,627.3 $ (106.7 ) 212 $ 1,548.6 $ (33.1 )
Non Credit Portion Of Other Than Temporary Impairment Recorded In Accumulated Other Comprehensive Income Table [Table Text Block]	The following table shows the total non-credit portion of the OTTI recorded in accumulated other comprehensive income, reflecting the original non-credit loss at the time the credit impairment was determined (i.e., unadjusted for valuation changes subsequent to the original write-down): March 31, December 31, (millions) 2017 2016 Fixed maturities: Residential mortgage-backed securities $ (43.3 ) $ (43.3 ) $ (43.3 ) Commercial mortgage-backed securities (0.5 ) (0.6 ) (0.6 ) Total fixed maturities $ (43.8 ) $ (43.9 ) $ (43.9 )
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table Text Block]	The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended March 31, 2017 and 2016 , for which a portion of the OTTI losses were also recognized in accumulated other comprehensive income at the time the credit impairments were determined and recognized: Three Months Ended March 31, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Reductions for securities sold/matured 0 (0.3 ) (0.3 ) Change in recoveries of future cash flows expected to be collected 1 0.3 0 0.3 Balance at March 31, 2017 $ 11.4 $ 0.1 $ 11.5 Three Months Ended March 31, 2016 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2015 $ 12.4 $ 0.4 $ 12.8 Reductions for securities sold/matured 0 0 0 Change in recoveries of future cash flows expected to be collected 1 (0.3 ) 0 (0.3 ) Balance at March 31, 2016 $ 12.1 $ 0.4 $ 12.5 1 Reflects the current period change in the expected recovery of prior impairments that will be accreted into income over the remaining life of the security.
Realized Gain (Loss) on Investments [Table Text Block]	The components of net realized gains (losses) for the three months ended March 31, were: Three Months (millions) 2017 2016 Gross realized gains on security sales Fixed maturities: U.S. government obligations $ 0.4 $ 14.3 State and local government obligations 0.5 10.9 Corporate and other debt securities 4.1 12.5 Residential mortgage-backed securities 0.1 0.9 Commercial mortgage-backed securities 1.2 3.6 Other asset-backed securities 0.3 0 Redeemable preferred stocks 0.3 0 Total fixed maturities 6.9 42.2 Equity securities: Nonredeemable preferred stocks 45.4 1.6 Common equities 7.4 9.5 Subtotal gross realized gains on security sales 59.7 53.3 Gross realized losses on security sales Fixed maturities: U.S. government obligations (3.2 ) (0.4 ) State and local government obligations (0.1 ) (0.1 ) Corporate and other debt securities (0.9 ) (0.4 ) Commercial mortgage-backed securities (2.4 ) (2.7 ) Total fixed maturities (6.6 ) (3.6 ) Equity securities: Nonredeemable preferred stocks (1.2 ) (1.0 ) Common equities (0.1 ) (4.9 ) Subtotal gross realized losses on security sales (7.9 ) (9.5 ) Net realized gains (losses) on security sales Fixed maturities: U.S. government obligations (2.8 ) 13.9 State and local government obligations 0.4 10.8 Corporate and other debt securities 3.2 12.1 Residential mortgage-backed securities 0.1 0.9 Commercial mortgage-backed securities (1.2 ) 0.9 Other asset-backed securities 0.3 0 Redeemable preferred stocks 0.3 0 Total fixed maturities 0.3 38.6 Equity securities: Nonredeemable preferred stocks 44.2 0.6 Common equities 7.3 4.6 Subtotal net realized gains (losses) on security sales 51.8 43.8 Other-than-temporary impairment losses Equity securities: Common equities (1.0 ) 0 Subtotal other-than-temporary impairment losses (1.0 ) 0 Other gains (losses) Hybrid securities 0.8 (0.7 ) Derivative instruments 0 (25.7 ) Litigation settlements 0.3 0 Subtotal other gains (losses) 1.1 (26.4 ) Total net realized gains (losses) on securities $ 51.9 $ 17.4
Components of Net Investment Income	The components of net investment income for the three months ended March 31, were: Three Months (millions) 2017 2016 Fixed maturities: U.S. government obligations $ 12.7 $ 4.8 State and local government obligations 13.3 13.5 Foreign government obligations 0.1 0.1 Corporate debt securities 29.6 28.2 Residential mortgage-backed securities 9.4 12.2 Agency residential pass-through obligations 0.2 0.4 Commercial mortgage-backed securities 18.8 20.9 Other asset-backed securities 9.9 6.0 Redeemable preferred stocks 3.4 3.8 Total fixed maturities 97.4 89.9 Equity securities: Nonredeemable preferred stocks 11.5 11.8 Common equities 13.3 14.5 Short-term investments 7.0 2.6 Investment income 129.2 118.8 Investment expenses (5.6 ) (4.8 ) Net investment income $ 123.6 $ 114.0
Schedule of Derivative Instruments [Table Text Block]	The following table shows the status of our derivative instruments at March 31, 2017 and 2016 , and December 31, 2016 , and for the three months ended March 31, 2017 and 2016 : (millions) Balance Sheet 2 Comprehensive Income Statement Assets (Liabilities) Pretax Net Realized Gains (Losses) Notional Value 1 Three Months Ended March 31, Dec. 31, March 31, Dec. 31, March 31, Derivatives designated as: 2017 2016 2016 Purpose Classification 2017 2016 2016 2017 2016 Hedging instrument Open: Interest rate swap $ 850 $ 0 $ 0 Manage Other assets $ 4.0 $ 0 $ 0 $ 0 $ 0 Closed: Ineffective cash flow hedge 31 0 370 Manage interest rate risk NA 0 0 0 0 0 Non-hedging instruments Liabilities: Interest rate swaps 0 565 0 Manage portfolio duration Other liabilities 0 (18.7 ) 0 0 (24.0 ) Closed: Interest rate swaps 0 185 750 Manage portfolio duration NA 0 0 0 0 (1.9 ) U.S. Treasury Note futures 0 55 135 Manage NA 0 0 0 0 0.2 Total NA NA NA $ 4.0 $ (18.7 ) $ 0 $ 0 $ (25.7 ) NA= Not applicable 1 The amounts represent the value held at quarter and year end for open positions and the maximum amount held during the period for closed positions. 2 To the extent we hold both derivative assets and liabilities with the same counterparty that are subject to an enforceable master netting arrangement, we reported them on a gross basis on our balance sheets, consistent with our historical presentation.

Fair Value (Tables)

Fair Value (Tables)	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Composition of Investment Portfolio by Major Security Type	The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2017 Fixed maturities: U.S. government obligations $ 3,325.8 $ 0 $ 0 $ 3,325.8 $ 3,345.5 State and local government obligations 0 2,488.4 0 2,488.4 2,475.8 Foreign government obligations 22.9 0 0 22.9 22.9 Corporate debt securities 0 5,208.0 0 5,208.0 5,199.6 Subtotal 3,348.7 7,696.4 0 11,045.1 11,043.8 Asset-backed securities: Residential mortgage-backed 0 1,409.6 0 1,409.6 1,393.9 Agency residential pass-through obligations 0 38.5 0 38.5 39.1 Commercial mortgage-backed 0 2,213.5 0 2,213.5 2,216.1 Other asset-backed 0 2,564.9 0 2,564.9 2,562.4 Subtotal asset-backed securities 0 6,226.5 0 6,226.5 6,211.5 Redeemable preferred stocks: Financials 0 63.2 0 63.2 60.4 Utilities 0 31.4 0 31.4 30.6 Industrials 0 108.1 0 108.1 98.5 Subtotal redeemable preferred stocks 0 202.7 0 202.7 189.5 Total fixed maturities 3,348.7 14,125.6 0 17,474.3 17,444.8 Equity securities: Nonredeemable preferred stocks: Financials 131.1 678.7 0 809.8 710.3 Subtotal nonredeemable preferred stocks 131.1 678.7 0 809.8 710.3 Common equities: Common stocks 2,977.8 0 0 2,977.8 1,451.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,977.8 0 0.3 2,978.1 1,451.5 Total fixed maturities and equity securities 6,457.6 14,804.3 0.3 21,262.2 19,606.6 Short-term investments 2,226.7 814.1 0 3,040.8 3,040.8 Total portfolio $ 8,684.3 $ 15,618.4 $ 0.3 $ 24,303.0 $ 22,647.4 Debt $ 0 $ 3,198.3 $ 121.1 $ 3,319.4 $ 3,111.7 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2016 Fixed maturities: U.S. government obligations $ 1,388.0 $ 0 $ 0 $ 1,388.0 $ 1,375.8 State and local government obligations 0 2,508.2 0 2,508.2 2,452.0 Foreign government obligations 21.9 0 0 21.9 21.9 Corporate debt securities 0 3,840.0 0 3,840.0 3,795.2 Subtotal 1,409.9 6,348.2 0 7,758.1 7,644.9 Asset-backed securities: Residential mortgage-backed 0 1,801.5 0 1,801.5 1,810.9 Agency residential pass-through obligations 0 46.6 0 46.6 46.5 Commercial mortgage-backed 0 2,416.7 9.5 2,426.2 2,418.7 Other asset-backed 0 1,703.7 0 1,703.7 1,706.3 Subtotal asset-backed securities 0 5,968.5 9.5 5,978.0 5,982.4 Redeemable preferred stocks: Financials 0 91.7 0 91.7 81.8 Utilities 0 51.6 0 51.6 65.1 Industrials 0 87.0 0 87.0 118.1 Subtotal redeemable preferred stocks 0 230.3 0 230.3 265.0 Total fixed maturities 1,409.9 12,547.0 9.5 13,966.4 13,892.3 Equity securities: Nonredeemable preferred stocks: Financials 153.8 678.4 0 832.2 731.9 Subtotal nonredeemable preferred stocks 153.8 678.4 0 832.2 731.9 Common equities: Common stocks 2,704.7 0 0 2,704.7 1,523.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,704.7 0 0.3 2,705.0 1,524.0 Total fixed maturities and equity securities 4,268.4 13,225.4 9.8 17,503.6 16,148.2 Short-term investments 3,730.0 96.9 0 3,826.9 3,826.9 Total portfolio $ 7,998.4 $ 13,322.3 $ 9.8 $ 21,330.5 $ 19,975.1 Debt $ 0 $ 2,789.2 $ 158.1 $ 2,947.3 $ 2,701.6 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2016 Fixed maturities: U.S. government obligations $ 2,870.1 $ 0 $ 0 $ 2,870.1 $ 2,899.2 State and local government obligations 0 2,502.6 0 2,502.6 2,509.5 Foreign government obligations 24.5 0 0 24.5 24.5 Corporate debt securities 0 4,550.9 0 4,550.9 4,557.8 Subtotal 2,894.6 7,053.5 0 9,948.1 9,991.0 Asset-backed securities: Residential mortgage-backed 0 1,458.7 0 1,458.7 1,448.5 Agency residential pass-through obligations 0 40.6 0 40.6 41.2 Commercial mortgage-backed 0 2,253.1 0.3 2,253.4 2,266.9 Other asset-backed 0 2,351.1 0 2,351.1 2,350.7 Subtotal asset-backed securities 0 6,103.5 0.3 6,103.8 6,107.3 Redeemable preferred stocks: Financials 0 59.5 0 59.5 59.8 Utilities 0 30.9 0 30.9 30.5 Industrials 0 101.5 0 101.5 98.5 Subtotal redeemable preferred stocks 0 191.9 0 191.9 188.8 Total fixed maturities 2,894.6 13,348.9 0.3 16,243.8 16,287.1 Equity securities: Nonredeemable preferred stocks: Financials 138.1 715.4 0 853.5 734.2 Subtotal nonredeemable preferred stocks 138.1 715.4 0 853.5 734.2 Common equities: Common stocks 2,812.0 0 0 2,812.0 1,437.1 Other risk investments 0 0 0.4 0.4 0.4 Subtotal common equities 2,812.0 0 0.4 2,812.4 1,437.5 Total fixed maturities and equity securities 5,844.7 14,064.3 0.7 19,909.7 18,458.8 Short-term investments 3,009.3 563.6 0 3,572.9 3,572.9 Total portfolio $ 8,854.0 $ 14,627.9 $ 0.7 $ 23,482.6 $ 22,031.7 Debt $ 0 $ 3,188.5 $ 127.3 $ 3,315.8 $ 3,148.2
Summary of Changes in Fair Value Associated With Level 3 Assets	The following tables provide a summary of changes in fair value associated with Level 3 assets for the three months ended March 31, 2017 and 2016 : Level 3 Fair Value Three Months Ended March 31, 2017 (millions) Fair Value at December 31, 2016 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain) Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at March 31, 2017 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 $ (0.3 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 Total fixed maturities 0.3 (0.3 ) 0 0 0 0 0 0 Equity securities: Common equities: Other risk investments 0.4 (0.1 ) 0 0 0 0 0 0.3 Total Level 3 securities $ 0.7 $ (0.4 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0.3 Level 3 Fair Value Three Months Ended March 31, 2016 (millions) Fair Value at December 31, 2015 Calls/ Purchases Sales Net Realized (Gain) Loss on Sales Change in Net Fair Value at March 31, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.9 $ (0.3 ) $ 0 $ 0 $ 0 $ (0.1 ) $ 0 $ 9.5 Total fixed maturities 9.9 (0.3 ) 0 0 0 (0.1 ) 0 9.5 Equity securities: Common equities: Other risk investments 0.3 0 0 0 0 0 0 0.3 Total Level 3 securities $ 10.2 $ (0.3 ) $ 0 $ 0 $ 0 $ (0.1 ) $ 0 $ 9.8
Summary of Quantitative Information about Level 3 Fair Value Measurements	The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at March 31, 2017 and 2016 , and December 31, 2016 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2017 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0 NA NA NA Subtotal Level 3 securities 0 Pricing exemption securities 1 0.3 Total Level 3 securities $ 0.3 NA= Not applicable, since we did not hold any commercial mortgage-backed Level 3 securities at March 31, 2017 . 1 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2016 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.5 External vendor Prepayment rate 1 0 % Subtotal Level 3 securities 9.5 Pricing exemption securities 2 0.3 Total Level 3 securities $ 9.8 1 Assumes that one security has 0% of the principal amount of the underlying loans that will be paid off prematurely in each year. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2016 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 External vendor Prepayment rate 1 0 % Subtotal Level 3 securities 0.3 Pricing exemption securities 2 0.4 Total Level 3 securities $ 0.7 1 Assumes that one security has 0% of the principal amount of the underlying loans that will be paid off prematurely in each year. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us.

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2017
Debt Table	Debt at each of the balance sheet periods consisted of: March 31, 2017 March 31, 2016 December 31, 2016 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 498.5 $ 527.9 $ 498.2 $ 538.4 $ 498.4 $ 528.8 2.45% Senior Notes due 2027 495.8 468.6 0 0 495.8 464.6 6 5/8% Senior Notes due 2029 296.0 386.3 295.8 388.7 295.9 380.1 6.25% Senior Notes due 2032 395.2 512.1 395.0 509.7 395.2 499.0 4.35% Senior Notes due 2044 346.5 366.7 346.4 377.9 346.4 362.3 3.70% Senior Notes due 2045 395.1 380.0 395.0 388.6 395.1 372.5 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 563.5 556.7 613.1 585.9 594.1 581.2 Other debt instruments 121.1 121.1 158.1 158.1 127.3 127.3 Total $ 3,111.7 $ 3,319.4 $ 2,701.6 $ 2,947.3 $ 3,148.2 $ 3,315.8
Other debt instruments
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block]	The other debt instruments reported in the table above represent ARX indebtedness and consist of: March 31, 2017 March 31, 2016 December 31, 2016 ($ in millions) Type of debt instrument Number of Instruments Carrying Value Number of Instruments Carrying Number of Instruments Carrying Stated Maturity Date(s) Term loans 2 $ 55.9 2 $ 80.8 2 $ 62.1 December 2018 and 2019 Junior subordinated notes 1 2 41.2 2 41.2 2 41.2 June 2036 and 2037 Senior notes 4 24.0 4 24.0 4 24.0 Various 2 Surplus note 3 0 0 1 12.1 0 0 November 2021 Total $ 121.1 $ 158.1 $ 127.3 1 ARX issued junior subordinated floating rate notes to trusts established by ARX in connection with issuances of trust preferred securities by the trusts (discussed below). 2 The senior notes mature in May 2033, April 2034, December 2034, and June 2035. 3 The surplus note was debt of the subsidiary disposed of by ARX in the exchange transaction during the second quarter 2016.

Loss and Loss Adjustment Expe25

Loss and Loss Adjustment Expense Reserves (Tables)	3 Months Ended
	Mar. 31, 2017
Loss and Loss Adjustment Expenses Reserves [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block]	Activity in the loss and loss adjustment expense reserves is summarized as follows: March 31, (millions) 2017 2016 Balance, Beginning of period $ 11,368.0 $ 10,039.0 Less reinsurance recoverables on unpaid losses 1,801.0 1,442.7 Net balance, Beginning of period 9,567.0 8,596.3 Incurred related to: Current year 4,152.2 3,853.0 Prior years 111.2 60.4 Total incurred 4,263.4 3,913.4 Paid related to: Current year 1,933.2 1,864.5 Prior years 2,118.7 1,867.2 Total paid 4,051.9 3,731.7 Net balance, End of period 9,778.5 8,778.0 Plus reinsurance recoverables on unpaid losses 1,850.4 1,508.7 Balance, End of period $ 11,628.9 $ 10,286.7

Supplemental Cash Flow Inform26

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information Table	We paid the following in the respective periods: Three Months Ended March 31, (millions) 2017 2016 Income taxes $ 0.7 $ 30.0 Interest 33.1 28.1

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2017
Segment Reporting [Abstract]
Reconciliation of Revenue and operating Income from Segments to Consolidated	Following are the operating results for the respective periods: Three Months Ended March 31, 2017 2016 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 2,631.5 $ 249.7 $ 2,347.9 $ 171.4 Direct 2,523.7 175.3 2,220.7 79.5 Total Personal Lines 1 5,155.2 425.0 4,568.6 250.9 Commercial Lines 645.5 67.4 548.8 61.1 Property 2 226.0 7.8 200.0 (24.5 ) Other indemnity 0 (0.2 ) 0 (0.7 ) Total underwriting operations 6,026.7 500.0 5,317.4 286.8 Fees and other revenues 3 85.2 NA 78.9 NA Service businesses 28.5 2.6 25.0 3.4 Investments 4 181.1 175.5 136.2 131.4 Gains on extinguishment of debt 0.2 0.2 0 0 Interest expense NA (36.8 ) NA (34.2 ) Consolidated total $ 6,321.7 $ 641.5 $ 5,557.5 $ 387.4 NA = Not applicable 1 Personal auto insurance accounted for 93% and 92% of the total Personal Lines segment net premiums earned in the first quarters of 2017 and 2016 , respectively; insurance for our special lines products (e.g., motorcycles, watercraft, and RVs) accounted for the balance of the Personal Lines net premiums earned. 2 For both the three months ended March 31, 2017 and 2016, pretax profit (loss) also includes $15.5 million of amortization expense predominately associated with the acquisition of a controlling interest in ARX. 3 Pretax profit (loss) for fees and other revenues are attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expenses.
Underwriting Margins and Combined Ratios for our Underwriting Operations	Following are the underwriting margins and combined ratios for our underwriting operations for the respective periods: Three Months Ended March 31, 2017 2016 Under-writing Combined Ratio Under-writing Combined Ratio Personal Lines Agency 9.5 % 90.5 7.3 % 92.7 Direct 6.9 93.1 3.6 96.4 Total Personal Lines 8.2 91.8 5.5 94.5 Commercial Lines 10.4 89.6 11.1 88.9 Property 1 3.4 96.6 (12.2) 112.2 Other indemnity 2 NM NM NM NM Total underwriting operations 8.3 91.7 5.4 94.6 1 Included in the three months ended March 31, 2017 and 2016, is 6.9 points and 7.8 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX. 2 Underwriting margins and combined ratios are not meaningful (NM) for our other indemnity businesses due to the low level of premiums earned by, and the variability of loss costs in, such businesses.

Dividends Dividends (Tables)

Dividends Dividends (Tables)	3 Months Ended
	Mar. 31, 2017
Dividends [Abstract]
Dividends	Following is a summary of our shareholder dividends that were declared in the last two years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Share Total 1 Annual – Variable December 2016 February 2017 $ 0.6808 $ 395.4 Annual – Variable December 2015 February 2016 $ 0.8882 $ 519.2 1 Based on an estimate of shares outstanding as of the record date. For the dividends declared in December 2016 and 2015, we paid $ 395.4 million and $519.0 million , respectively.

Other Comprehensive Income (L29

Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2017
Equity [Abstract]
Components of Other Comprehensive Income (Loss)	The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net Net Foreign (Income)loss attributable to NCI Balance at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Other comprehensive income (loss) before reclassifications: Investment securities 254.5 (89.2 ) 165.3 165.3 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions 4.0 (1.4 ) 2.6 0 2.6 0 0 Foreign currency translation adjustment 0.5 (0.2 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) (1.5 ) 0.6 (0.9 ) 0 0 0 (0.9 ) Total other comprehensive income (loss) before reclassifications 257.5 (90.2 ) 167.3 165.3 2.6 0.3 (0.9 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (1.0 ) 0.4 (0.6 ) (0.6 ) 0 0 0 Net realized gains (losses) on securities 51.6 (18.1 ) 33.5 33.5 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 51.1 (17.9 ) 33.2 32.9 0.3 0 0 Total other comprehensive income (loss) 206.4 (72.3 ) 134.1 132.4 2.3 0.3 (0.9 ) Balance at March 31, 2017 $ 1,645.9 $ (578.4 ) $ 1,067.5 $ 1,072.0 $ (7.1 ) $ (0.8 ) $ 3.4 Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at December 31, 2015 $ 1,234.5 $ (434.1 ) $ 800.4 $ 809.0 $ (8.2 ) $ (1.5 ) $ 1.1 Other comprehensive income (loss) before reclassifications: Investment securities 142.7 (50.3 ) 92.4 92.4 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes (0.1 ) 0.1 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.7 (0.2 ) 0.5 0 0 0.5 0 Loss attributable to noncontrolling interest (NCI) (3.3 ) 1.2 (2.1 ) 0 0 0 (2.1 ) Total other comprehensive income (loss) before reclassifications 140.0 (49.2 ) 90.8 92.4 0 0.5 (2.1 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings 0 0 0 0 0 0 0 Net realized gains (losses) on securities 35.3 (12.4 ) 22.9 22.9 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 35.8 (12.6 ) 23.2 22.9 0.3 0 0 Total other comprehensive income (loss) 104.2 (36.6 ) 67.6 69.5 (0.3 ) 0.5 (2.1 ) Balance at March 31, 2016 $ 1,338.7 $ (470.7 ) $ 868.0 $ 878.5 $ (8.5 ) $ (1.0 ) $ (1.0 )

Redeemable Noncontrolling Int30

Redeemable Noncontrolling Interest (Tables)	3 Months Ended
	Mar. 31, 2017
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	The changes in the components of redeemable NCI during the three months ended March 31, 2017 and 2016 , and the year ended December 31, 2016 , were: (millions) March 31, 2017 March 31, 2016 December 31, 2016 Balance, Beginning of period $ 483.7 $ 464.9 $ 464.9 Net income attributable to NCI 6.0 0.5 26.2 Other comprehensive income (loss) attributable to NCI 0.9 2.1 (3.2 ) Exercise of employee stock options 3.4 0 0 Change in redemption value of NCI 0.2 (0.1 ) (4.2 ) Balance, End of period $ 494.2 $ 467.4 $ 483.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Net Carrying Amount of Other Intangible Assets	The following table is a summary of the net carrying amount of other intangible assets as of March 31, 2017 and 2016 , and December 31, 2016 : (millions) March 31, 2017 March 31, 2016 December 31, 2016 Intangible assets subject to amortization $ 404.9 $ 467.0 $ 420.4 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 417.3 $ 479.4 $ 432.8 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented.
Intangible Assets Subject to Amortization	Intangible assets subject to amortization consisted of the following: (millions) March 31, 2017 March 31, 2016 December 31, 2016 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 73.2 $ 183.0 $ 256.2 $ 36.6 $ 219.6 $ 256.2 $ 64.1 $ 192.1 Agency relationships 159.2 22.7 136.5 159.2 11.4 147.8 159.2 19.9 139.3 Software rights 79.1 21.5 57.6 79.1 10.8 68.3 79.1 18.8 60.3 Trade name 34.8 7.0 27.8 34.8 3.5 31.3 34.8 6.1 28.7 Total $ 529.3 $ 124.4 $ 404.9 $ 529.3 $ 62.3 $ 467.0 $ 529.3 $ 108.9 $ 420.4 Amortization expense was $15.5 million for the three months ended March 31, 2017 and 2016

Basis of Presentation Basis of

Basis of Presentation Basis of Presentation-Additional Information (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Business Acquisition [Line Items]
Assets Held-for-sale, Property, Plant and Equipment	$ 0	$ 8,700,000	$ 8,700,000
ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage	69.00%	69.20%	69.20%

Investments Investment Portfoli

Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 22,647.4	$ 22,031.7	$ 19,975.1
Gross Unrealized Gains	1,748.1	1,588.9	1,493.2
Gross Unrealized Losses	(95.1)	(139.8)	(138.1)
Net Realized Gains (Losses)	2.6	1.8	0.3
Fair Value	$ 24,303	$ 23,482.6	$ 21,330.5
% of Total Fair Value	100.00%	100.00%	100.00%
Other Payables to Broker-Dealers and Clearing Organizations	$ 111	$ 27.8	$ 47.3
Securities Held In Consolidated Non Insurance Subsidiary	700	1,300	700
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost	17,444.8	16,287.1	13,892.3
Gross Unrealized Gains	105.4	76.5	181.8
Gross Unrealized Losses	(78.5)	(121.6)	(108)
Net Realized Gains (Losses)	2.6	1.8	0.3
Fair Value	$ 17,474.3	$ 16,243.8	$ 13,966.4
% of Total Fair Value	71.90%	69.20%	65.50%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 3,345.5	$ 2,899.2	$ 1,375.8
Gross Unrealized Gains	0.9	0	12.2
Gross Unrealized Losses	(20.6)	(29.1)	0
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 3,325.8	$ 2,870.1	$ 1,388
% of Total Fair Value	13.70%	12.20%	6.50%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,475.8	$ 2,509.5	$ 2,452
Gross Unrealized Gains	23.3	13.8	57.4
Gross Unrealized Losses	(10.7)	(20.7)	(1.2)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 2,488.4	$ 2,502.6	$ 2,508.2
% of Total Fair Value	10.20%	10.70%	11.80%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 22.9	$ 24.5	$ 21.9
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 22.9	$ 24.5	$ 21.9
% of Total Fair Value	0.10%	0.10%	0.10%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 5,199.6	$ 4,557.8	$ 3,795.2
Gross Unrealized Gains	23.1	17.3	50.6
Gross Unrealized Losses	(15)	(24.3)	(6.2)
Net Realized Gains (Losses)	0.3	0.1	0.4
Fair Value	$ 5,208	$ 4,550.9	$ 3,840
% of Total Fair Value	21.40%	19.40%	18.00%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,393.9	$ 1,448.5	$ 1,810.9
Gross Unrealized Gains	24.6	23.7	19.5
Gross Unrealized Losses	(11)	(15)	(28.5)
Net Realized Gains (Losses)	2.1	1.5	(0.4)
Fair Value	$ 1,409.6	$ 1,458.7	$ 1,801.5
% of Total Fair Value	5.80%	6.20%	8.40%
Fixed maturities | Agency residential pass-through obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 39.1	$ 41.2	$ 46.5
Gross Unrealized Gains	0.1	0	0.2
Gross Unrealized Losses	(0.7)	(0.6)	(0.1)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 38.5	$ 40.6	$ 46.6
% of Total Fair Value	0.20%	0.20%	0.20%
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,216.1	$ 2,266.9	$ 2,418.7
Gross Unrealized Gains	13.7	12	26.4
Gross Unrealized Losses	(16.3)	(25.5)	(18.9)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 2,213.5	$ 2,253.4	$ 2,426.2
% of Total Fair Value	9.10%	9.60%	11.40%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,562.4	$ 2,350.7	$ 1,706.3
Gross Unrealized Gains	5.4	4.6	1.3
Gross Unrealized Losses	(3.1)	(4.4)	(4.2)
Net Realized Gains (Losses)	0.2	0.2	0.3
Fair Value	$ 2,564.9	$ 2,351.1	$ 1,703.7
% of Total Fair Value	10.60%	10.00%	8.00%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 189.5	$ 188.8	$ 265
Gross Unrealized Gains	14.3	5.1	14.2
Gross Unrealized Losses	(1.1)	(2)	(48.9)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 202.7	$ 191.9	$ 230.3
% of Total Fair Value	0.80%	0.80%	1.10%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 710.3	$ 734.2	$ 731.9
Gross Unrealized Gains	111.3	135.4	124.7
Gross Unrealized Losses	(11.8)	(16.1)	(24.4)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 809.8	$ 853.5	$ 832.2
% of Total Fair Value	3.30%	3.60%	3.90%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,451.5	$ 1,437.5	$ 1,524
Gross Unrealized Gains	1,531.4	1,377	1,186.7
Gross Unrealized Losses	(4.8)	(2.1)	(5.7)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 2,978.1	$ 2,812.4	$ 2,705
% of Total Fair Value	12.30%	12.00%	12.70%
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 3,040.8	$ 3,572.9	$ 3,826.9
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 3,040.8	$ 3,572.9	$ 3,826.9
% of Total Fair Value	12.50%	15.20%	17.90%
Designated as Hedging Instruments | Interest Rate Swap [Member]
Schedule of Available-for-sale Securities [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 136.1

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Hybrid Securities [Line Items]
Hybrid securities	$ 239.4	$ 219.5	$ 273.7
Fixed maturities
Hybrid Securities [Line Items]
Hybrid securities	239.4	219.5	224.2
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	44.5	40.1	39.2
Fixed maturities | Residential mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	186.6	170.5	174.4
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	8.3	8.9	10.6
Equity securities | Nonredeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 0	$ 0	$ 49.5

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) $ in Millions	Mar. 31, 2017USD ($)
Available for sale, cost
Less than one year	$ 4,370.5
One to five years	9,988.3
Five to ten years	3,030.3
Ten years or greater	55.7
Total	17,444.8
Available for sale, Fair value
Less than one year	4,382.3
One to five years	9,995.8
Five to ten years	3,030
Ten years or greater	66.2
Total	$ 17,474.3

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Mar. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security	Mar. 31, 2016USD ($)security
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1,056	1,369	670
Total Fair Value	$ 10,135.4	$ 11,175.9	$ 5,360.4
Gross Unrealized Losses	$ (95.1)	$ (139.8)	$ (138.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	872	1,157	338
Less than 12 Months Fair Value	$ 8,999.6	$ 9,627.3	$ 2,450.2
Less than 12 Months Unrealized Losses	$ (69.2)	$ (106.7)	$ (34.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	184	212	332
12 Months or Greater Fair Value	$ 1,135.8	$ 1,548.6	$ 2,910.2
12 Months or Greater Unrealized Losses	$ (25.9)	$ (33.1)	$ (103.9)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	979	1,281	584
Total Fair Value	$ 9,889.2	$ 10,824.2	$ 4,928.6
Gross Unrealized Losses	$ (78.5)	$ (121.6)	$ (108)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	806	1,080	267
Less than 12 Months Fair Value	$ 8,872.8	$ 9,432.4	$ 2,286.9
Less than 12 Months Unrealized Losses	$ (63.7)	$ (101.2)	$ (26.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	173	201	317
12 Months or Greater Fair Value	$ 1,016.4	$ 1,391.8	$ 2,641.7
12 Months or Greater Unrealized Losses	$ (14.8)	$ (20.4)	$ (81.9)
Fixed maturities | U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	34	30	5
Total Fair Value	$ 2,747	$ 2,774	$ 74.9
Gross Unrealized Losses	$ (20.6)	$ (29.1)	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	34	30	5
Less than 12 Months Fair Value	$ 2,747	$ 2,774	$ 74.9
Less than 12 Months Unrealized Losses	$ (20.6)	$ (29.1)	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	0	0	0
12 Months or Greater Fair Value	$ 0	$ 0	$ 0
12 Months or Greater Unrealized Losses	$ 0	$ 0	$ 0
Fixed maturities | State and local government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	340	618	112
Total Fair Value	$ 986.3	$ 1,497.9	$ 267.8
Gross Unrealized Losses	$ (10.7)	$ (20.7)	$ (1.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	310	584	63
Less than 12 Months Fair Value	$ 906.5	$ 1,404.3	$ 132.8
Less than 12 Months Unrealized Losses	$ (9.5)	$ (19.6)	$ (0.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	30	34	49
12 Months or Greater Fair Value	$ 79.8	$ 93.6	$ 135
12 Months or Greater Unrealized Losses	$ (1.2)	$ (1.1)	$ (0.9)
Fixed maturities | Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	147	184	60
Total Fair Value	$ 2,494.7	$ 2,615.1	$ 672.3
Gross Unrealized Losses	$ (15)	$ (24.3)	$ (6.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	139	175	21
Less than 12 Months Fair Value	$ 2,415.1	$ 2,559.9	$ 169.2
Less than 12 Months Unrealized Losses	$ (14.9)	$ (24)	$ (0.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	8	9	39
12 Months or Greater Fair Value	$ 79.6	$ 55.2	$ 503.1
12 Months or Greater Unrealized Losses	$ (0.1)	$ (0.3)	$ (5.8)
Fixed maturities | Residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	182	178	157
Total Fair Value	$ 880.9	$ 917.7	$ 1,345.1
Gross Unrealized Losses	$ (11)	$ (15)	$ (28.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	82	69	52
Less than 12 Months Fair Value	$ 264.4	$ 175.8	$ 426.9
Less than 12 Months Unrealized Losses	$ (1.1)	$ (1.1)	$ (3.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	100	109	105
12 Months or Greater Fair Value	$ 616.5	$ 741.9	$ 918.2
12 Months or Greater Unrealized Losses	$ (9.9)	$ (13.9)	$ (24.6)
Fixed maturities | Agency residential pass-through obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	65	55	25
Total Fair Value	$ 35.4	$ 36	$ 9.1
Gross Unrealized Losses	$ (0.7)	$ (0.6)	$ (0.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	56	48	3
Less than 12 Months Fair Value	$ 32.7	$ 33.9	$ 0.4
Less than 12 Months Unrealized Losses	$ (0.6)	$ (0.6)	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	9	7	22
12 Months or Greater Fair Value	$ 2.7	$ 2.1	$ 8.7
12 Months or Greater Unrealized Losses	$ (0.1)	$ 0	$ (0.1)
Fixed maturities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	92	111	130
Total Fair Value	$ 1,131.9	$ 1,347.3	$ 1,185.4
Gross Unrealized Losses	$ (16.3)	$ (25.5)	$ (18.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	73	85	49
Less than 12 Months Fair Value	$ 985.7	$ 1,061.2	$ 395.4
Less than 12 Months Unrealized Losses	$ (14.2)	$ (22.9)	$ (2.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	19	26	81
12 Months or Greater Fair Value	$ 146.2	$ 286.1	$ 790
12 Months or Greater Unrealized Losses	$ (2.1)	$ (2.6)	$ (16.7)
Fixed maturities | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	118	103	86
Total Fair Value	$ 1,601.6	$ 1,605.2	$ 1,175.2
Gross Unrealized Losses	$ (3.1)	$ (4.4)	$ (4.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	112	89	70
Less than 12 Months Fair Value	$ 1,521.4	$ 1,423.3	$ 999.3
Less than 12 Months Unrealized Losses	$ (2.8)	$ (3.9)	$ (3.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	6	14	16
12 Months or Greater Fair Value	$ 80.2	$ 181.9	$ 175.9
12 Months or Greater Unrealized Losses	$ (0.3)	$ (0.5)	$ (1)
Fixed maturities | Redeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1	2	9
Total Fair Value	$ 11.4	$ 31	$ 198.8
Gross Unrealized Losses	$ (1.1)	$ (2)	$ (48.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	0	0	4
Less than 12 Months Fair Value	$ 0	$ 0	$ 88
Less than 12 Months Unrealized Losses	$ 0	$ 0	$ (16.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	1	2	5
12 Months or Greater Fair Value	$ 11.4	$ 31	$ 110.8
12 Months or Greater Unrealized Losses	$ (1.1)	$ (2)	$ (32.8)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	77	88	86
Total Fair Value	$ 246.2	$ 351.7	$ 431.8
Gross Unrealized Losses	$ (16.6)	$ (18.2)	$ (30.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	66	77	71
Less than 12 Months Fair Value	$ 126.8	$ 194.9	$ 163.3
Less than 12 Months Unrealized Losses	$ (5.5)	$ (5.5)	$ (8.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	11	11	15
12 Months or Greater Fair Value	$ 119.4	$ 156.8	$ 268.5
12 Months or Greater Unrealized Losses	$ (11.1)	$ (12.7)	$ (22)
Equity securities | Nonredeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	9	13	13
Total Fair Value	$ 197	$ 329.6	$ 344.4
Gross Unrealized Losses	$ (11.8)	$ (16.1)	$ (24.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	5	8	4
Less than 12 Months Fair Value	$ 78.5	$ 175.2	$ 79
Less than 12 Months Unrealized Losses	$ (0.7)	$ (3.8)	$ (2.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	4	5	9
12 Months or Greater Fair Value	$ 118.5	$ 154.4	$ 265.4
12 Months or Greater Unrealized Losses	$ (11.1)	$ (12.3)	$ (21.8)
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	68	75	73
Total Fair Value	$ 49.2	$ 22.1	$ 87.4
Gross Unrealized Losses	$ (4.8)	$ (2.1)	$ (5.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	61	69	67
Less than 12 Months Fair Value	$ 48.3	$ 19.7	$ 84.3
Less than 12 Months Unrealized Losses	$ (4.8)	$ (1.7)	$ (5.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	7	6	6
12 Months or Greater Fair Value	$ 0.9	$ 2.4	$ 3.1
12 Months or Greater Unrealized Losses	$ 0	$ (0.4)	$ (0.2)

Investments Total Non-Credit Po

Investments Total Non-Credit Portion of Other-Than-Temporary Impairment Recorded in Accumulated Other Comprehensive Income, Reflecting Original Non-Credit Loss at Time Credit Impairment (Detail) - Fixed maturities - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	$ (43.8)	$ (43.9)	$ (43.9)
Residential mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	(43.3)	(43.3)	(43.3)
Commercial mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	$ (0.5)	$ (0.6)	$ (0.6)

Investments OTTI Credit Losses

Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 11.5	$ 12.8
Reductions for securities sold/matured	(0.3)	0
Change in recoveries of future cash flows expected to be collected	0.3	(0.3)
Ending balance	11.5	12.5
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	11.1	12.4
Reductions for securities sold/matured	0	0
Change in recoveries of future cash flows expected to be collected	0.3	(0.3)
Ending balance	11.4	12.1
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.4	0.4
Reductions for securities sold/matured	(0.3)	0
Change in recoveries of future cash flows expected to be collected	0	0
Ending balance	$ 0.1	$ 0.4

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	$ 59.7	$ 53.3
Gross realized losses on securities sales	(7.9)	(9.5)
Net realized gains (losses) on securities sales	51.8	43.8
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(1)	0
Other gains (losses)	1.1	(26.4)
Total net realized gains (losses) on securities	51.9	17.4
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	6.9	42.2
Gross realized losses on securities sales	(6.6)	(3.6)
Net realized gains (losses) on securities sales	0.3	38.6
Fixed maturities | U.S. government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.4	14.3
Gross realized losses on securities sales	(3.2)	(0.4)
Net realized gains (losses) on securities sales	(2.8)	13.9
Fixed maturities | State and local government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.5	10.9
Gross realized losses on securities sales	(0.1)	(0.1)
Net realized gains (losses) on securities sales	0.4	10.8
Fixed maturities | Corporate and other debt securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	4.1	12.5
Gross realized losses on securities sales	(0.9)	(0.4)
Net realized gains (losses) on securities sales	3.2	12.1
Fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.1	0.9
Net realized gains (losses) on securities sales	0.1	0.9
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	1.2	3.6
Gross realized losses on securities sales	(2.4)	(2.7)
Net realized gains (losses) on securities sales	(1.2)	0.9
Fixed maturities | Other asset-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.3	0
Available For Sale Securities Net Realized Gain Loss Excluding Other Than Temporary Impairment Losses And Net Holding Period Gains Losses	0.3	0
Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.3	0
Net realized gains (losses) on securities sales	0.3	0
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	45.4	1.6
Gross realized losses on securities sales	(1.2)	(1)
Net realized gains (losses) on securities sales	44.2	0.6
Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	7.4	9.5
Gross realized losses on securities sales	(0.1)	(4.9)
Net realized gains (losses) on securities sales	7.3	4.6
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(1)	0
Hybrid and Derivative Instruments and Litigation Settlements | Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	0.3	0
Hybrid and Derivative Instruments and Litigation Settlements | Hybrid Preferred Stock
Gain (Loss) on Investments [Line Items]
Other gains (losses)	0.8	(0.7)
Hybrid and Derivative Instruments and Litigation Settlements | Derivative Instruments
Gain (Loss) on Investments [Line Items]
Other gains (losses)	$ 0	$ (25.7)

Investments Components of Net I

Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Net Investment Income [Line Items]
Investment income	$ 129.2	$ 118.8
Investment expenses	(5.6)	(4.8)
Net Investment Income	123.6	114
Fixed maturities
Net Investment Income [Line Items]
Investment income	97.4	89.9
Fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Investment income	12.7	4.8
Fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Investment income	13.3	13.5
Fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Investment income	0.1	0.1
Fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Investment income	29.6	28.2
Fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Investment income	9.4	12.2
Fixed maturities | Agency residential pass-through obligations
Net Investment Income [Line Items]
Investment income	0.2	0.4
Fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Investment income	18.8	20.9
Fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Investment income	9.9	6
Fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Investment income	3.4	3.8
Equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Investment income	11.5	11.8
Equity securities | Common equities
Net Investment Income [Line Items]
Investment income	13.3	14.5
Short-term investments
Net Investment Income [Line Items]
Investment income	$ 7	$ 2.6

Investments Derivative Instrume

Investments Derivative Instruments (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Aug. 25, 2016
Derivative [Line Items]
Balance Sheet - Fair Value	$ 4,000,000	$ (18,700,000)	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	0	(25,700,000)
Designated as Hedging Instruments | Ineffective Cash Flow Hedge [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 31,000,000	0	$ 370,000,000	$ 350,000,000
Purpose	Manage  interest  rate risk		Manage interest rate risk
Balance Sheet - Fair Value	$ 0	0	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	0	0
Designated as Hedging Instruments | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0	0	0
Purpose	Manage  interest  rate risk
Balance Sheet - Fair Value	$ 4,000,000	0	0
Income Statement - Net Realized Gain (Loss) on Securities	0	0
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Balance Sheet - Fair Value	0		0
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Other Liabilities [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	$ 565,000,000	0
Purpose		Manage  portfolio duration
Balance Sheet - Fair Value	0	$ (18,700,000)	0
Income Statement - Net Realized Gain (Loss) on Securities	0	(24,000,000)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Closed Positions [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	$ 185,000,000	$ 750,000,000
Purpose		Manage  portfolio duration	Manage portfolio duration
Balance Sheet - Fair Value	0	$ 0	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	0	(1,900,000)
Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative [Line Items]
Balance Sheet - Fair Value	0	0	0
Not Designated as Hedging Instrument [Member] | Future [Member] | Closed Positions [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	$ 55,000,000	$ 135,000,000
Purpose		Manage  portfolio duration	Manage portfolio duration
Balance Sheet - Fair Value	0	$ 0	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	$ 0	$ 200,000

Investments Investments - Addit

Investments Investments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2017USD ($)security	Mar. 31, 2016USD ($)security	Dec. 31, 2016USD ($)security	Aug. 25, 2016USD ($)	Jun. 18, 2007
Schedule of Investments [Line Items]
Balance Sheet - Fair Value	$ 4,000,000	$ (18,700,000)	$ 0
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase	0	0
Securities Sold under Agreements to Repurchase	0	0	0
Open reverse repurchase commitments	0	0	0
Proceeds from Securities Purchased under Agreements to Resell	0
Largest Outstanding Balance of Reverse Repurchase Agreements		$ 265,000,000
RRP Largest Outstanding Balance Number of Days Open		1 day
Reverse Repurchase Agreements, Total Days of Activity		10 days
Reverse Repurchase Agreement Average Daily Balance		$ 160,700,000
Gross unrealized losses	$ 95,100,000	$ 138,100,000	$ 139,800,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1,056	670	1,369
Trading securities	$ 0	$ 0	$ 0
Net realized gains (losses) on securities	52,900,000	17,400,000
Income Statement - Net Realized Gain (Loss) on Securities	$ 0	(25,700,000)
2.45% Senior Notes due 2027
Schedule of Investments [Line Items]
Debt, face amount				$ 500,000,000
Debt, stated interest rate	2.45%			2.45%
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Schedule of Investments [Line Items]
Debt, stated interest rate	6.70%				6.70%
Fixed Income Securities
Schedule of Investments [Line Items]
Gross unrealized losses	$ 90,300,000
Equity securities
Schedule of Investments [Line Items]
Gross unrealized losses	$ 16,600,000	$ 30,100,000	$ 18,200,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	77	86	88
Equity securities | Common equities
Schedule of Investments [Line Items]
Gross unrealized losses	$ 4,800,000	$ 5,700,000	$ 2,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	68	73	75
Equity securities | Nonredeemable preferred stocks
Schedule of Investments [Line Items]
Gross unrealized losses	$ 11,800,000	$ 24,400,000	$ 16,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	9	13	13
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline	6.00%
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses	$ 78,500,000	$ 108,000,000	$ 121,600,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	979	584	1,281
Fixed maturities | Corporate debt securities
Schedule of Investments [Line Items]
Percentage which the issuer receives the security back at once the change of control is triggered	1.00%
Realized investment losses	$ 3,100,000
Gross unrealized losses	$ 15,000,000	$ 6,200,000	$ 24,300,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	147	60	184
Fixed maturities | Redeemable preferred stocks
Schedule of Investments [Line Items]
Gross unrealized losses	$ 1,100,000	$ 48,900,000	$ 2,000,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1	9	2
Categories of Investments, Marketable Securities, Trading Securities
Schedule of Investments [Line Items]
Net realized gains (losses) on securities	$ 0	$ 0
Russell One Thousand | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio	96.00%
Managed Equity Strategy | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio	4.00%
Designated as Hedging Instruments | Ineffective Cash Flow Hedge [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value	$ 0	0	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	0	0
Derivative, Notional Amount	31,000,000	0	370,000,000	$ 350,000,000
Designated as Hedging Instruments | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value	4,000,000	0	0
Income Statement - Net Realized Gain (Loss) on Securities	0	0
Derivative, Notional Amount	0	0	0
Unrealized gain (loss) on interest rate cash flow hedges	4,000,000
Derivative, Collateral, Right to Reclaim Cash	$ 136,100,000
Derivative, Term of Contract	30 years
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value	$ 0		0
Derivative, Collateral, Right to Reclaim Cash		20,600,000
Not Designated as Hedging Instrument [Member] | Future [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value	0	0	0
Closed Positions [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value	0	0	0
Income Statement - Net Realized Gain (Loss) on Securities	0	(1,900,000)
Derivative, Notional Amount	0	185,000,000	750,000,000
Closed Positions [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value	0	0	0
Income Statement - Net Realized Gain (Loss) on Securities	0	200,000
Derivative, Notional Amount	$ 0	$ 55,000,000	$ 135,000,000

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 24,303	$ 23,482.6	$ 21,330.5
Long-Term Debt, Fair Value	3,319.4	3,315.8	2,947.3
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	17,474.3	16,243.8	13,966.4
Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	11,045.1	9,948.1	7,758.1
Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	6,226.5	6,103.8	5,978
Fixed maturities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,325.8	2,870.1	1,388
Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,325.8	2,870.1	1,388
Fixed maturities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,488.4	2,502.6	2,508.2
Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,488.4	2,502.6	2,508.2
Fixed maturities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	22.9	24.5	21.9
Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	22.9	24.5	21.9
Fixed maturities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,208	4,550.9	3,840
Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,208	4,550.9	3,840
Fixed maturities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,409.6	1,458.7	1,801.5
Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,409.6	1,458.7	1,801.5
Fixed maturities | Agency residential pass-through obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	38.5	40.6	46.6
Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	38.5	40.6	46.6
Fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,213.5	2,253.4	2,426.2
Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,213.5	2,253.4	2,426.2
Fixed maturities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,564.9	2,351.1	1,703.7
Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,564.9	2,351.1	1,703.7
Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	202.7	191.9	230.3
Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	63.2	59.5	91.7
Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	31.4	30.9	51.6
Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	108.1	101.5	87
Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	809.8	853.5	832.2
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	809.8	853.5	832.2
Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,978.1	2,812.4	2,705
Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,977.8	2,812	2,704.7
Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.4	0.3
Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	21,262.2	19,909.7	17,503.6
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,040.8	3,572.9	3,826.9
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	22,647.4	22,031.7	19,975.1
Debt	3,111.7	3,148.2	2,701.6
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	17,444.8	16,287.1	13,892.3
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	11,043.8	9,991	7,644.9
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	6,211.5	6,107.3	5,982.4
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,345.5	2,899.2	1,375.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,475.8	2,509.5	2,452
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	22.9	24.5	21.9
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,199.6	4,557.8	3,795.2
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,393.9	1,448.5	1,810.9
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	39.1	41.2	46.5
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,216.1	2,266.9	2,418.7
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,562.4	2,350.7	1,706.3
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	189.5	188.8	265
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	60.4	59.8	81.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	30.6	30.5	65.1
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	98.5	98.5	118.1
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	710.3	734.2	731.9
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	710.3	734.2	731.9
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,451.5	1,437.5	1,524
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,451.2	1,437.1	1,523.7
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.4	0.3
Carrying (Reported) Amount, Fair Value Disclosure | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	19,606.6	18,458.8	16,148.2
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,040.8	3,572.9	3,826.9
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	8,684.3	8,854	7,998.4
Long-Term Debt, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,348.7	2,894.6	1,409.9
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,348.7	2,894.6	1,409.9
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,325.8	2,870.1	1,388
Fair Value, Inputs, Level 1 | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	22.9	24.5	21.9
Fair Value, Inputs, Level 1 | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	131.1	138.1	153.8
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	131.1	138.1	153.8
Fair Value, Inputs, Level 1 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,977.8	2,812	2,704.7
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,977.8	2,812	2,704.7
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	6,457.6	5,844.7	4,268.4
Fair Value, Inputs, Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,226.7	3,009.3	3,730
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	15,618.4	14,627.9	13,322.3
Long-Term Debt, Fair Value	3,198.3	3,188.5	2,789.2
Fair Value, Inputs, Level 2 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	14,125.6	13,348.9	12,547
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	7,696.4	7,053.5	6,348.2
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	6,226.5	6,103.5	5,968.5
Fair Value, Inputs, Level 2 | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 2 | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,488.4	2,502.6	2,508.2
Fair Value, Inputs, Level 2 | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 2 | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,208	4,550.9	3,840
Fair Value, Inputs, Level 2 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,409.6	1,458.7	1,801.5
Fair Value, Inputs, Level 2 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	38.5	40.6	46.6
Fair Value, Inputs, Level 2 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,213.5	2,253.1	2,416.7
Fair Value, Inputs, Level 2 | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,564.9	2,351.1	1,703.7
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	202.7	191.9	230.3
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	63.2	59.5	91.7
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	31.4	30.9	51.6
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	108.1	101.5	87
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	678.7	715.4	678.4
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	678.7	715.4	678.4
Fair Value, Inputs, Level 2 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 2 | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	14,804.3	14,064.3	13,225.4
Fair Value, Inputs, Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	814.1	563.6	96.9
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.7	9.8
Long-Term Debt, Fair Value	121.1	127.3	158.1
Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0.3	9.5
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0.3	9.5
Fair Value, Inputs, Level 3 | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0.3	9.5
Fair Value, Inputs, Level 3 | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.4	0.3
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.4	0.3
Fair Value, Inputs, Level 3 | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.7	9.8
Fair Value, Inputs, Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 0	$ 0	$ 0

Fair Value Summary of Changes i

Fair Value Summary of Changes in Fair Value Associated With Level 3 Assets (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	$ 0.7	$ 10.2
Calls/ Maturities/ Paydowns	(0.4)	(0.3)
Purchases	0	0
Sales	0	0
Net Realized (Gain) Loss on Sales	0	0
Change in Valuation	0	(0.1)
Net Transfers in (out)	0	0
Ending Fair value	0.3	9.8
Equity securities | Common equities | Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.4	0.3
Calls/ Maturities/ Paydowns	(0.1)	0
Purchases	0	0
Sales	0	0
Net Realized (Gain) Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers in (out)	0	0
Ending Fair value	0.3	0.3
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.3	9.9
Calls/ Maturities/ Paydowns	(0.3)	(0.3)
Purchases	0	0
Sales	0	0
Net Realized (Gain) Loss on Sales	0	0
Change in Valuation	0	(0.1)
Net Transfers in (out)	0	0
Ending Fair value	0	9.5
Asset-backed Securities | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.3	9.9
Calls/ Maturities/ Paydowns	(0.3)	(0.3)
Purchases	0	0
Sales	0	0
Net Realized (Gain) Loss on Sales	0	0
Change in Valuation	0	(0.1)
Net Transfers in (out)	0	0
Ending Fair value	$ 0	$ 9.5

Fair Value Summary of Quantitat

Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2016USD ($)securityInvestment	Dec. 31, 2016USD ($)securityInvestment	Mar. 31, 2017USD ($)Investment
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	$ 21,330,500,000	$ 23,482,600,000	$ 24,303,000,000
Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	832,200,000	853,500,000	809,800,000
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	832,200,000	853,500,000	809,800,000
Fixed maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	13,966,400,000	16,243,800,000	17,474,300,000
Fixed maturities | Residential mortgage-backed securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	1,801,500,000	1,458,700,000	1,409,600,000
Fixed maturities | Commercial mortgage-backed securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	2,426,200,000	2,253,400,000	2,213,500,000
Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	9,800,000	700,000	300,000
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	300,000	400,000	300,000
Fair Value, Inputs, Level 3 | Subtotal Level 3 Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	$ 9,500,000	$ 300,000	$ 0
Fair Value, Inputs, Level 3 | Internal Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | Investment	0	0	0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	$ 0	$ 0	$ 0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	$ 9,500,000	$ 300,000	0
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | External Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | security	1	1
Available-for-sale Securities	$ 9,500,000	$ 300,000	$ 0
Valuation Technique	External vendor	External vendor
Unobservable Input, prepayment rate	0.00%	0.00%

Fair Value Fair Value - Additio

Fair Value Fair Value - Additional Information (Detail)	Mar. 31, 2017USD ($)Investment	Dec. 31, 2016USD ($)securityInvestment	Mar. 31, 2016USD ($)securityInvestment
Fair Value Measurements [Line Items]
Fair Value	$ 24,303,000,000	$ 23,482,600,000	$ 21,330,500,000
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 0	$ 0	$ 0
Quoted prices percentage of fair value assets	100.00%	100.00%	100.00%
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Fair Value	$ 8,684,300,000	$ 8,854,000,000	$ 7,998,400,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Fair Value	15,618,400,000	14,627,900,000	13,322,300,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	300,000	700,000	9,800,000
Fixed maturities
Fair Value Measurements [Line Items]
Fair Value	$ 17,474,300,000	$ 16,243,800,000	$ 13,966,400,000
Quoted prices percentage of fair value assets	71.90%	69.20%	65.50%
Fixed maturities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Fair Value	$ 3,348,700,000	$ 2,894,600,000	$ 1,409,900,000
Fixed maturities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Fair Value	14,125,600,000	13,348,900,000	12,547,000,000
Fixed maturities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	$ 0	$ 300,000	$ 9,500,000
Internal Pricing | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities | Investment	0	0	0
Dealer Quoted Pricing | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Quoted prices percentage of fair value assets	1.00%	1.00%	2.00%
Vendor Quoted Pricing | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Quoted prices percentage of fair value assets	54.00%	52.00%	37.00%
Vendor Quoted Pricing | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Quoted prices percentage of fair value assets	99.00%	99.00%	98.00%
Commercial mortgage-backed securities | Fixed maturities
Fair Value Measurements [Line Items]
Fair Value	$ 2,213,500,000	$ 2,253,400,000	$ 2,426,200,000
Quoted prices percentage of fair value assets	9.10%	9.60%	11.40%
Commercial mortgage-backed securities | External Pricing | Fixed maturities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	$ 0	$ 300,000	$ 9,500,000
Number of securities | security		1	1

Debt (Detail)

Debt (Detail)		Mar. 31, 2017USD ($)instrument	Dec. 31, 2016USD ($)instrument	Mar. 31, 2016USD ($)instrument
Debt Instrument [Line Items]
Fair Value		$ 3,319,400,000	$ 3,315,800,000	$ 2,947,300,000
Debt, Long-term and Short-term, Combined Amount	[1]	3,111,700,000	3,148,200,000	2,701,600,000
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Carrying Value		498,500,000	498,400,000	498,200,000
Fair Value		527,900,000	528,800,000	538,400,000
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Carrying Value		495,800,000	495,800,000	0
Fair Value		468,600,000	464,600,000	0
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Carrying Value		395,200,000	395,200,000	395,000,000
Fair Value		512,100,000	499,000,000	509,700,000
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Carrying Value		296,000,000	295,900,000	295,800,000
Fair Value		386,300,000	380,100,000	388,700,000
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Carrying Value		346,500,000	346,400,000	346,400,000
Fair Value		366,700,000	362,300,000	377,900,000
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Carrying Value		395,100,000	395,100,000	395,000,000
Fair Value		380,000,000	372,500,000	388,600,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Carrying Value		563,500,000	594,100,000	613,100,000
Fair Value		556,700,000	581,200,000	585,900,000
Other debt instruments
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 121,100,000	$ 127,300,000	$ 158,100,000
Notes Payable, Other Payables [Member] | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		2	2	2
Debt, Long-term and Short-term, Combined Amount		$ 55,900,000	$ 62,100,000	$ 80,800,000
Trust Preferred Debt Securities [Member] | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		2	2	2
Carrying Value		$ 41,200,000	$ 41,200,000	$ 41,200,000
Senior Notes [Member] | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		4	4	4
Carrying Value		$ 24,000,000	$ 24,000,000	$ 24,000,000
Surplus Note [Member] | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		0	0	1
Debt, Long-term and Short-term, Combined Amount		$ 0	$ 0	$ 12,100,000
[1]	Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Aug. 25, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Jun. 18, 2007
Debt Instrument [Line Items]
Gains (losses) on extinguishment of debt		$ 200,000	$ 0
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 100,000,000		$ 100,000,000
Debt term		30 days
Borrowings outstanding		$ 0		0
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Debt, stated interest rate		3.75%
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Debt, stated interest rate	2.45%	2.45%
Debt, face amount	$ 500,000,000
Proceeds, net of underwriter's discounts and commissions	$ 495,600,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Repurchase of debt, face amount		$ 30,900,000	0	19,800,000
Debt, stated interest rate		6.70%			6.70%
Gains (losses) on extinguishment of debt		$ 200,000		1,600,000
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Debt, stated interest rate		6.625%
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Debt, stated interest rate		6.25%
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Debt, stated interest rate		4.35%
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Debt, stated interest rate		3.70%
Other Assets [Member]
Debt Instrument [Line Items]
Ownership interest in the variable interest entities		$ 1,300,000	$ 1,300,000	$ 1,300,000
Federal Funds Rate [Member] | Discretionary Line of Credit
Debt Instrument [Line Items]
Basis spread on variable interest rate		0.50%		0.50%

Income Taxes Additional Informa

Income Taxes Additional Information (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Valuation allowance	$ 0	$ 0	$ 0

Loss and Loss Adjustment Expe50

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 11,628.9	$ 10,286.7	$ 11,368	$ 10,039
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	1,850.4	1,508.7	1,801	1,442.7
Liability for Unpaid Claims and Claims Adjustment Expense, Net	9,778.5	8,778	$ 9,567	$ 8,596.3
Current Year Claims and Claims Adjustment Expense	4,152.2	3,853
Prior Year Claims and Claims Adjustment Expense	111.2	60.4
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	4,263.4	3,913.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	1,933.2	1,864.5
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	2,118.7	1,867.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	4,051.9	3,731.7
Short-duration Insurance Contracts, Accident Year 2016 [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	100
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		46
Personal Lines | Direct Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	37	11
Personal Lines | Personal Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	124
Personal Lines | Agency Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 68	32
Commercial Lines | Property and Casualty, Liability, Commercial Insurance Product Line [Domain]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 15

Supplemental Cash Flow Inform51

Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Income taxes	$ 0.7	$ 30
Interest	$ 33.1	$ 28.1

Segment Information Operating R

Segment Information Operating Results (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 6,321,700,000	$ 5,557,500,000
Pretax Profit (Loss)	641,500,000	387,400,000
Amortization of intangible assets	15,500,000	15,500,000
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	2,631,500,000	2,347,900,000
Pretax Profit (Loss)	249,700,000	171,400,000
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	2,523,700,000	2,220,700,000
Pretax Profit (Loss)	175,300,000	79,500,000
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,026,700,000	5,317,400,000
Pretax Profit (Loss)	500,000,000	286,800,000
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	5,155,200,000	4,568,600,000
Pretax Profit (Loss)	$ 425,000,000	$ 250,900,000
Personal auto insurance percentage of the total personal lines segment net premiums earned	93.00%	92.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 645,500,000	$ 548,800,000
Pretax Profit (Loss)	67,400,000	61,100,000
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	226,000,000	200,000,000
Pretax Profit (Loss)	7,800,000	(24,500,000)
Amortization of intangible assets	15,500,000	15,500,000
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0	0
Pretax Profit (Loss)	(200,000)	(700,000)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	85,200,000	78,900,000
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	28,500,000	25,000,000
Pretax Profit (Loss)	2,600,000	3,400,000
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	181,100,000	136,200,000
Pretax Profit (Loss)	175,500,000	131,400,000
Segment Reconciling Items | Gains on extinguishment of debt
Segment Reporting Information [Line Items]
Revenues	200,000	0
Pretax Profit (Loss)	200,000	0
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (36,800,000)	$ (34,200,000)

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Operating Segments - Underwriting operations	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Underwriting Margin	8.30%	5.40%
Combined Ratio	91.70%	94.60%
Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	8.20%	5.50%
Combined Ratio	91.80%	94.50%
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	9.50%	7.30%
Combined Ratio	90.50%	92.70%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	6.90%	3.60%
Combined Ratio	93.10%	96.40%
Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	10.40%	11.10%
Combined Ratio	89.60%	88.90%
Property
Segment Reporting Information [Line Items]
Underwriting Margin	3.40%	(12.20%)
Combined Ratio	96.60%	112.20%
Combined ratio amortization points	0.069	0.078

Dividends Dividends (Details)

Dividends Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Payments of Ordinary Dividends, Common Stock	$ 395.4	$ 519
Dividends payable			$ 0	$ 395.4	$ 0	$ 519.2
Annual variable dividend [Member]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.6808	$ 0.8882

Dividends - Additional Informat

Dividends - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2017USD ($)
Dividends [Abstract]
Target percentage used to determine annual variable dividend	33.33%
Gainshare factor used to determine annual variable dividend range, minimum	0
Minimum Payments of Ordinary Dividends, Common Stock	$ 0
Gainshare factor used to determine annual variable dividend range, maximum	2
Gainshare factor used to determine annual variable dividend	1.83

Other Comprehensive Income (L56

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pre tax total accumulated other comprehensive income, beginning balance	$ 1,439.5	$ 1,234.5	$ 1,234.5
Investment securities, pre tax	254.5	142.7
Net non-credit related OTTI losses, adjusted for valuation changes, pre tax	0	(0.1)
Forecasted transactions, pre tax	4	0
Foreign currency translation adjustment, pre tax	0.5	0.7
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	(1.5)	(3.3)
Total other comprehensive income (loss) before reclassifications, pre tax	257.5	140
Total reclassification adjustment for amounts realized in net income, pre tax	51.1	35.8
Total other comprehensive income (loss), pre tax	206.4	104.2
Pre tax total accumulated other comprehensive income, ending balance	1,645.9	1,338.7	1,439.5
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Accumulated other comprehensive income loss tax (Provision) benefit, beginning balance	(506.1)	(434.1)	(434.1)
Investment securities, Total tax (provision) benefit	(89.2)	(50.3)
Net non-credit related OTTI losses, adjusted for valuation changes, Total tax (provision) benefit	0	0.1
Forecasted transactions, Total tax (provision) benefit	(1.4)	0
Foreign currency translation adjustment, Total tax (provision) benefit	(0.2)	(0.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	0.6	1.2
Total other comprehensive income (loss) before reclassifications, Total tax (provision) benefit	(90.2)	(49.2)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(17.9)	(12.6)
Total other comprehensive income (loss), Total tax (provision) benefit	(72.3)	(36.6)
Accumulated other comprehensive income loss tax (Provision) benefit, ending balance	(578.4)	(470.7)	(506.1)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	933.4	800.4	800.4
Investment securities, after tax	165.3	92.4
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0	0
Forecasted transactions, after tax	2.6	0
Foreign currency translation adjustment, after tax	0.3	0.5
Other comprehensive (income) loss attributable to NCI	(0.9)	(2.1)	3.2
Total other comprehensive income (loss) before reclassifications, after tax	167.3	90.8
Total reclassification adjustment for amounts realized in net income, after tax	33.2	23.2
Other comprehensive income (loss)	134.1	67.6
After tax total accumulated other comprehensive income, ending balance	1,067.5	868	933.4
Net realized gains (losses) on securities	52.9	17.4
Interest expense	36.8	34.2
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	939.6	809	809
Investment securities, after tax	165.3	92.4
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0	0
Forecasted transactions, after tax	0	0
Foreign currency translation adjustment, after tax	0	0
Other comprehensive (income) loss attributable to NCI	0	0
Total other comprehensive income (loss) before reclassifications, after tax	165.3	92.4
Total reclassification adjustment for amounts realized in net income, after tax	32.9	22.9
Other comprehensive income (loss)	132.4	69.5
After tax total accumulated other comprehensive income, ending balance	1,072	878.5	939.6
Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(9.4)	(8.2)	(8.2)
Investment securities, after tax	0	0
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0	0
Forecasted transactions, after tax	2.6	0
Foreign currency translation adjustment, after tax	0	0
Other comprehensive (income) loss attributable to NCI	0	0
Total other comprehensive income (loss) before reclassifications, after tax	2.6	0
Total reclassification adjustment for amounts realized in net income, after tax	0.3	0.3
Other comprehensive income (loss)	2.3	(0.3)
After tax total accumulated other comprehensive income, ending balance	(7.1)	(8.5)	(9.4)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(1.1)	(1.5)	(1.5)
Investment securities, after tax	0	0
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0	0
Forecasted transactions, after tax	0	0
Foreign currency translation adjustment, after tax	0.3	0.5
Other comprehensive (income) loss attributable to NCI	0	0
Total other comprehensive income (loss) before reclassifications, after tax	0.3	0.5
Total reclassification adjustment for amounts realized in net income, after tax	0	0
Other comprehensive income (loss)	0.3	0.5
After tax total accumulated other comprehensive income, ending balance	(0.8)	(1)	(1.1)
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	4.3	1.1	1.1
Investment securities, after tax	0	0
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0	0
Forecasted transactions, after tax	0	0
Foreign currency translation adjustment, after tax	0	0
Other comprehensive (income) loss attributable to NCI	(0.9)	(2.1)
Total other comprehensive income (loss) before reclassifications, after tax	(0.9)	(2.1)
Total reclassification adjustment for amounts realized in net income, after tax	0	0
Other comprehensive income (loss)	(0.9)	(2.1)
After tax total accumulated other comprehensive income, ending balance	3.4	(1)	$ 4.3
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income pretax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	(1)	0
Net realized gains (losses) on securities	51.6	35.3
Interest expense	0.5	0.5
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0.4	0
Net realized gains (losses) on securities	(18.1)	(12.4)
Interest expense	(0.2)	(0.2)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income after tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	(0.6)	0
Net realized gains (losses) on securities	33.5	22.9
Interest expense	0.3	0.3
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	(0.6)	0
Net realized gains (losses) on securities	33.5	22.9
Interest expense	0	0
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0	0
Net realized gains (losses) on securities	0	0
Interest expense	0.3	0.3
Reclassification out of Accumulated Other Comprehensive Income | Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0	0
Net realized gains (losses) on securities	0	0
Interest expense	0	0
Reclassification out of Accumulated Other Comprehensive Income | Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0	0
Net realized gains (losses) on securities	0	0
Interest expense	$ 0	$ 0

Other Comprehensive Income (L57

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement, Additional Information (Detail) $ in Millions	Mar. 31, 2017USD ($)
Pre Tax
Components Of Other Comprehensive Income Loss [Line Items]
Net unrealized gains on forecasted transactions, expected to reclassify into income within the next 12 months	$ 0.4

Redeemable Noncontrolling Int58

Redeemable Noncontrolling Interest (Details) $ in Millions		3 Months Ended			12 Months Ended
		Mar. 31, 2017USD ($)shares		Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)	Apr. 01, 2021installment	Apr. 01, 2018installment	Apr. 01, 2015installment
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment								2
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance, Beginning of period		$ 483.7	[1]	$ 464.9	$ 464.9
Net income attributable to NCI		6		0.5	26.2
Other comprehensive income (loss) attributable to NCI		0.9		2.1	(3.2)
Exercise of employee stock options		3.4		0	0
Change in redemption value of NCI		0.2		(0.1)	(4.2)
Balance, End of period	[1]	$ 494.2		$ 467.4	$ 483.7
ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage		69.00%		69.20%	69.20%
Shares held in Employee Stock Option Plan, Suspense Shares | shares		22,550
Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value		$ 31.9
Scenario, Forecast [Member]
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment						1	1
Scenario, Forecast [Member] | ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage						100.00%	80.00%
[1]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Goodwill and Intangible Asset59

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Period Increase (Decrease)	$ 0	$ 0
Goodwill, Acquired During Period			$ 1,800,000
Goodwill, Impaired, Accumulated Impairment Loss	0
Goodwill	449,400,000	447,600,000	$ 449,400,000
Amortization of intangible assets	$ 15,500,000	$ 15,500,000

Goodwill and Intangible Asset60

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Schedule of Other Intangible Assets [Line Items]
Indefinite lived intangible assets	$ 12.4	$ 12.4	$ 12.4
Total	417.3	432.8	479.4
Accumulated amortization	125	$ 109.5	$ 62.9
Licensing Agreements [Member]
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	$ 0.6

Goodwill and Intangible Asset61

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 125	$ 109.5	$ 62.9
Policies in force [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	73.2	64.1	36.6
Net Carrying Amount	183	192.1	219.6
Agency relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	22.7	19.9	11.4
Net Carrying Amount	136.5	139.3	147.8
Software rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1	79.1
Accumulated Amortization	21.5	18.8	10.8
Net Carrying Amount	57.6	60.3	68.3
Trade name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8	34.8
Accumulated Amortization	7	6.1	3.5
Net Carrying Amount	27.8	28.7	31.3
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3	529.3
Accumulated Amortization	124.4	108.9	62.3
Net Carrying Amount	$ 404.9	$ 420.4	$ 467

New Accounting Standards Effect

New Accounting Standards Effect of Standard Change (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Item Effected [Line Items]
Provision for income taxes	$ 211.2	$ 128.7
Adjustments for New Accounting Pronouncement
Item Effected [Line Items]
Provision for income taxes	$ (8.3)

Subsequent Event (Details)

Subsequent Event (Details) - 4.125% Senior Notes due 2047 - Subsequent Event	Apr. 03, 2017USD ($)
Subsequent Event [Line Items]
Debt, face amount	$ 850,000,000
Debt, stated interest rate	4.125%
Proceeds, net of underwriter's discounts and commissions	$ 841,100,000
Additional fees related to issuance	1,500,000
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)